The '76 to mid-year '80 FMJ differentials are 58½” wide (wheel mounting surface to wheel mounting surface).
The mid-year '80 to '89 FMJ differentials are 59½” wide (wheel mounting surface to wheel mounting surface).
The '02-07 KJ (Liberty) differentials are 63-3/8” (63.375”) wide (wheel mounting surface to wheel mounting surface), which is either 2” or 2½” wider per side (depending on year FMJ one is working on). This is a measurement that I performed, with this differential being a possible solution. If a person selected wheels with a bit of offset to them, this differential is very doable as far as width goes.
A few differences with the '02-07 Liberty differential:
- There are no leaf spring perches. This differential uses coil springs – but there is no problem with cutting off the coil spring hardware (white arrows, below), getting a pair of 3” tube spring perches and getting them welded on.
AllStar Weld On Leaf Spring Perch Pad 3" Axle Housing For Ford Chevy Mopar | eBay is one source (and there are many other source for parts out there).
Note: pictures were swiped from the internet.
- The yoke will be different (blue arrow, above). All Chrysler made (North America) differential yokes made 1970 to present will interchange (with exception of Dana 70/80 (HD Truck) differentials).
FMJ yoke (29 spline 7260 U-joint
Liberty yoke
Companion flange
Mercedes design (300/Charger/Challenger) – which uses a rubber U-joint!
I'm not recommending anyone to change to the Mercedes design for the rubber will crack and fail in about 10 years, just like tires do/will (if car is driven, or not). I just used this as an example to show this yoke will fit onto your FMJ, if you had a desire to install one.
If a person is having a new propeller shaft made or old one modified (replacing a 7¼” with an 8¼”, then using the newer design flange type yoke makes some sense.
I've heard the U-joint distance is the same –
but have no first hand knowledge of this (using existing FMJ propeller shaft/joint/yoke vs. using existing FMJ propeller shaft/joint and using new style flat yoke/companion flange – if replacing existing 8¼” with a Liberty 8¼”).
- The Liberty differential has the machined surface for but is not drilled/tapped for using the pinion snubber (red arrow, above). This might not matter for a /6 or a 318 2-bbl driver, but is more important for those with more HP (now or later) than that.
- This is not a major concern for those not performing repairs to the Liberty differential, but might be. Most of the internal parts are the same with an FMJ 8¼” except for axle shaft length and axle shaft spline/spider gear spline count. The '68-95 8¼” use 27 spline axle shafts and '97 up 8¼” use 29 spline axle shafts/spider gears. The differential carriers (Limited Slip, or not) makes a difference so '96 (mid-year change) is where the change is made.
The (rear, not 4*4 front) differential gear sets will interchange from '68 to present.
I'm not opposed to using the Liberty 8¼”, just as long as you keep the above in mind. As time permits, I'm still measuring other (newer) Chrysler/Jeeps for other possibilities.
The rear disk brake setup will bolt right on FMJ's (except for early 7¼'s – which require a bit of modification, first). The differential gear sets will also bolt on to existing 8¼'s. The newer carriers (Limited Slip) – not so much, but the '96 and older ones will. I would guess that maybe 1 of 5 Liberty's have limited slip (which is only a guess)
BudW