By James Hasler
Part 1: The Ford 8.8 Diff
The Dana 35 axle in the rear of YJ and TJ Wranglers, MJ Comanches, XJ Cherokees and ZJ Grand Cherokees is not known to be up to the task of moderate four wheeling, let alone running "The Hammers". This is not to say that they never hold up, but 33" tires and lockers can lead to disaster if you like to get cozy with the stupid pedal. The weaknesses of the Dana 35 are it's thin axle tubes, weak axle shafts and weak ring and pinion gears when you start getting into the 4.56 and up range. Our good buddy Andrew snapped his right rear Dana 35 shaft with open diffs, 4.11's and 4 angry squirrels under the hood, so...
What are our options? You can beef up the 35 with a truss and 30 spline shafts or a full floater which might be fine for some. But this can still cost you $1000 or more. You can spend anywhere from $2000 to 4000 on a custom Dana 44, 60 or Ford 9". This sounds great as you'll have a bolt in diff, but it won't sound as great when you have to write the check.
Another option is to build up a diff from a junkyard. This takes a lot more "know-how" but is cheaper. Scout Dana 44s, certain Wagoneer Dana 44s or Early Ford Bronco 9" rears are about the right width, but they have the wrong bolt pattern. This usually leads to carrying two spare tires or having to upgrade the front to match.
If you can find the elusive Dana 44 under some 87-89 XJ Cherokees or MJ's, they fit the bill as they have the right bolt pattern and are exactly the right width. The problem is they are expensive ($800-1000 from my quotes) or flat out impossible to find. What's left?
Well, our good friends at Ford have been selling a bazillion Ford Explorer's since the 80's. Underneath these grocery getters lies the Ford 8.8 rear diff...the successor to the Ford 9" in vehicles up to 1/2 ton trucks. In 1995 to 2000 trim, the Ford Explorer 8.8 rear diff is roughly the same width as the Dana 35 (3/4-1" narrower), has enormous tubes (3.25" diameter), an 8.8" ring gear, 31 spline axle shafts, factory disk brakes and come with 3.55/3.73 or 4.11 gears and sometimes a Trac-Lok diff. (Pre-95 Ford 8.8's and those found in Rangers and mustangs usually come with 28 spline shafts and drum brakes.) I have heard that stock to stock, the Ford 8.8 is 120% stronger than the Dana 35. Also, the Ford 8.8 is stronger than a Dana 44 in every way and is only slightly heavier. (The Ford 8.8 has C-Clips, however, but you have to break the huge axle shafts first. C-clip eliminators and Warn Full Floater kits are available as well as Moser alloy shafts.) And the best part is that you can pick one of these up for as low as $350 bucks complete. How do I know? I just did!
I got the above diff for $350 which included good looking pads/rotors as well as the e-brake cables, hard brake lines and 3.55 gears on an open carrier. I've already cracked the cover and the gears, axle shafts and outer bearings/seals look good! I also took a pic of the Dana 35 27 spline shaft next to the Ford 8.8 31 spline shaft...
Wow! Big difference! The only way to get anything stronger would be to step up to a 1.5", 35 spline shaft in a Ford 9" or Dana 60!
The Ford 8.8 out of an Explorer has massive...and I mean MASSIVE axle tubes. I would gather that they have around 1/4" thick tubing (or more) with a 3.25" overall diameter!! Because of this, you need new spring plates and U-bolts.
I opted to also get brand new spring mounts and
shock mounts. Mountain
Off Road Enterprises (MORE) bundles the
spring plates, shock mounts, spring mounts and u-bolts 
into
a kit for $105 (Part Number 98500) I found out later that reusing the
Ford spring pads make more sense. See Part 4 for the
details.
So, you can probably find the other items cheaper if you looked for each piece individually. MORE, however, was pretty much the pioneers of the Ford 8.8 swap into YJs, so they have a wealth of information about the swap details.
Another item to get is a driveshaft flange to mate the flat flange on the Ford 8.8 to the 1310 series u-joints found on the YJ driveshaft. The flat flange is shown below:
You can see why there is no way to bolt the YJ driveshaft up to this! Anyway, the adapter is Spicer part number 2-2-1379. You can get this from MORE as well, but again, you'll pay for it.
The Ford 8.8 has roughly the same pinion length as the Dana 35 but it does appear to be somewhat lower. It is also offset somewhat (1-2") to the passenger side. With stock to 2.5" springs, you probably have a chance on getting your stock driveshaft to work. I am planning on 4.56 gears with 32-35" tires which leads to a relatively high driveshaft rpm on the highway compared to my 3.07's. I already have some driveshaft vibes with my 2.5" springs and 1" transfer case drop, so, I'm going with Advance Adapters Heavy Duty Slip Yoke Eliminator and CV driveshaft, which replaces the stock NP231 main shaft with a HUGE main shaft out of a NP241. (~50% more torque capacity.) I will not only get vibe free operation, but I'll be able to raise my transfer case back up (or go to a belly up type skid plate) and I will also be able to remove my driveshaft on the trail without draining my transfer case fluid.
Fortunately, the AA tail housing that comes with their kit is not the lovely blue shown above...Anyway, the SYE will also enable me to go to a higher spring lift in the future without dealing with any driveshaft issues other than reshimming the rear axle and possibly lengthening the driveshaft.
From what I've read and heard from MORE, the YJ brake system does not need adjustment to cope with the rear disk brakes. However, you will need to modify the something to get the rubber brake line going from the axle to the frame to work. The one on the Ford 8.8 is much too short if it were to mount to the stock frame location. I'm letting M.I.T.'s techs figure out what to do but I'll let you know what the final solution looks like.
Here's what the original brake lines look like on the housing. First, there is the passenger side adapter that converts the rubber line from the caliper to a hard line that goes up and over the pumpkin.
The driver's side adapter has a rubber line going to one caliper and a rubber line going to the frame. It also has a threaded socket which the hard line goes into from the passenger side caliper.
I'm hopefull that we won't have to mess with the hard lines on the housing.
As for the e-brakes, the stock Ford 8.8 e-brake cables should work nicely, but they don't easily. Grand Cherokee (ZJ) e-brake cables on models with rear disk brakes should work great, though, or you can simply purchase some cables from an axle shop such as MIT. Here's what the stock Ford 8.8 cables look like.
They would work except that they meet at the YJ junction block at different lengths. Some modifications could be used to make them work, however, with enough patience. If you can fabricate and you get the Ford cables cheap, it might be a good option. (I've got a set if anyone wants them.) I've also heard that Lokar (Part Number EC-81FU) has a kit for the e-brakes that runs around $80 but I'm still not sure what exactly the kit includes.
Finally, the Ford 8.8 is somewhat prone to bearing cap flex. These diffs are found behind V-8's in drag racing type situations (Mustangs) so it's probable that the bearing cap flex would not be a problem under a Jeep. However, the cover that came with my Ford 8.8 was completely rusty so I bought a bearing support cover for it. I chose a unit from LPW...their Ultimate 8.8 cover. It's around $140 bucks, but it's better than putting a cheapo chrome jobby on it. Besides the two Grade 8 bolts for putting pressure on the bearing caps, the cover somes with a drain and fill plug as well...
The Ultimate LPW Cover can be found at JEGs, but Granatelli, Ford Motorsports and Moser Engineering make similar bearing support covers.
So, here's a run down of what you really need:
Optionally, you might need or want:
Swapping this diff under a Grand Cherokee or other late model Jeep would also be totally feasible. The only difference would be in having to weld on control arm brackets instead of spring mounts.
The most important thing about an axle swap is that it serves your purpose by being as strong as you need as well as being affordable for what you end up with. I've already given you a taste as to what the other options run, so here's the run down on the Ford 8.8 cost when swapping it under a YJ.
Best Case: The average price around for an 8.8 in San Diego is roughly $500-600. This includes everything from brake lines to calipers and pads. If you wanted to run 3.73 or 4.11's with a Track-Lok limited slip, you could wait until you found an 8.8 with these options. If you can weld, you could simply buy the driveshaft flange and MORE spring kit for $150, buy a new brakeline to go from the diff to the frame and you've got a beefy rear axle swap done for maybe 700-800 bucks. Not too bad.
Most Likely Case: Most likely, you'd pick up an 8.8 with decent brakes but you'll want to regear it and drop in a locker. For a sake of comparison, we'll go with a Detroit Locker (~$500) and 4.56 gears (~$200 for gears and install kit). You pay someone $200 to regear it and install the locker and $150 to weld on the spring plates. You sort out the brake lines yourself so you're looking at a total of ~$1600.
Worst Case: You need to buy all new brake components (~$300), you need to regear (~$200 for gears and install kit) and you want a Detroit locker (~$500). You still need the flange ($45) and spring mount kit ($105) but you pay someone else to weld everything on (~$150) and install the gears/locker ($200). At this point you are close to $2000. I made a few calls around to axle builders and you could get a Dana 44 for a YJ with a Detroit and disc brakes for around $2300 or a Dana 60 for $2500.
So, the Ford 8.8 swap can be very cheap swap if you're content with what you get from Ford as original equipment and you pick a junkyard axle carefully, or it can simply be a $500 or so lower cost solution to a custom axle if you have to regear it, add a locker and replace worn out components.
Part 4: The Details...
As promised, here are the details to making the Ford 8.8 swap work for you. I hope you can learn from my minor misfortunes. Here's the beast under my YJ:
It's big, bad and ugly! I love it! I've driven on it for a few days and everything is vibe and leak free. The locker is, well...a Detroit! But it's behavior behind a 5-speed is to be expected. After a few days with it, it's not very noticeable any more.
So, I'll first start with the brake lines. Save money and cut-off and reuse the stock Ford parts. They have a provision for bolting the hard lines to them. (The MORE ones do not.) By reusing them, you can reuse the entire Ford hard lines. (I have a set of MORE spring pads if you really want them.) Here's the passenger side. If you've looked at a stock Ford 8.8, the brake lines will look exactly the same.
Here's the driver's side. Again, everything was kept where it was originally.
In fact, I even reused the Ford rubber line going from the diff to the frame. I was able to do this since a drop bracket was made to mate the hard line on the frame to the Ford hose. If you're running a lot of lift, you can use an extended line sold by makers of Ford Explorer suspension lift kits.
In the pictures, the rubber line looks like it is touching the shock but it is actually about an inch away. I need to flex it up to make sure nothing contacts, though.
As for braking performance, the rear discs make a noticeable difference in braking performace. However, some experimentation might be useful if you want to truly maximize your braking. Rear discs usually require more proportioning than drums, so even though they work great without any modifications, I bet they could work better.
The stock YJ breather hose was reused on the Ford diff. Below are some shots of the AA SYE and CV driveshaft as well as a frontal view of the hunk of iron residing on my rear leafsprings.
Damn that thing is big! Yippee!
Here's how the custom e-brake cables were routed ($40 each from MIT). The passenger side one goes up and over the muffler heat shield with plenty of slack near the wheel.
The driver's side has plenty of slack and mounts through the gas filter skid. It does have plenty of slack, but it could be relocated below the skid if necessary.
When getting the shock mounts welded on a spring under axle configuration, remember that the axle tube serves as the lower bumpstop. Since the Ford tubes are 3.25" versus the Dana 35 tubes of 2.5", you are essentially getting a 3/4" bumpstop extension with the swap. Since my shocks were previously set up for max compression (1/4" to spare), I raised my lower shock mounts 3/4" relative to their old position. So, even if I lose a bit of compression due to the tubes, I'll gain it back in droop. You can also use this opportunity to run longer shocks as well.
As for the LPW bearing support cover, well, it's not going to work. The stock cover has no room to spare. Anyone want to buy mine? I don't think it'll be necessary anyway. I think you'd need a spring-over (like MOREs) to run one of these covers or countersink the bearing cap bolts. At this point, I simply don't have the horsepower to deflect that monster ring gear.
I did not have ABS on my YJ, but I have had some questions from Cherokee owners who want to keep their ABS. The Ford 8.8 uses a tone ring within the pumpkin, whereas the Jeep system uses tone rings on the end of the axle shafts. However, Tom Zehrbach has gotten the ABS to work when he put a Ford 8.8 under his Grand Cherokee. Some of his comments:
"I got the ABS to work, but it was a huge pain. The shoulders of the Ford shafts (where the tone rings press on) are actually smaller than the shoulders of the Dana shafts. Unfortunately, the difference is too small for a sleeve, so you have to bore out the tone ring a bit so that a sleeve can be fabricated and fit between the ring and the shaft. With the tone ring on the shaft, the axle tube is 1/8" too long, you can't get the shaft in far enough to slip the C-clip in. A grinder fixes that problem. You also have to grind out an area on the flange for the tone ring sensor."
Finally, the Ford 8.8 sits about 3/4 to 1" lower than the Dana 35 from axle tube to bottom of pumpkin in case anyone is interested.
So, I'm thrilled to have gotten rid of my Dana 35 and so far, I'm loving everything that I've done. I think I've covered all the details here and hopefully saved you from making some of the little mistakes that I did (MORE spring mounts, LPW bearing support cover). It's a great rear axle swap and it turned out to be relatively painless in getting everything to mate.
Update:
I flexed everything up and things have turned out great. The raised shock mounts in combo with the larger tubes turned out great. Still have some shock compression to spare and I gained some droop. All of the brake/e-brake/breather cables/tubes worked great as well with no problems.