The truck I'm restoring has a 9.25, the parts truck a 8.75 and I believe a 8.25 from a Chrysler car behind the shed. The main difference is that the 9.25 has the larger, wider U joint compared to the others. However, I thought I read somewhere that the yoke will interchange between the different axles. I'm not sure, so you'd need to talk to someone more knowledgable than me.
Something else, many people dislike the 9.25, claiming it's weak, junk, etc. It has a crush sleeve that takes up play in the pinion gear and that sleeve wears or compresses, resulting in a lot of slop, which ain't good for a differential. Supposedly the other Chrysler differentials don't use the sleeve, although substitute sleeves can be used on the 9.25 that will eliminate the problem forever. It seems some people claim the 8.75 is super durable, better than the 9.25 and so on. All I know is that my 4-wheel drive parts truck had around 98,000 miles and was really put through the wringer by the previous owner and the 8.75 rear end has a LOT of slop in the pinion shaft, probably a good 30-40 degrees of rotation at the yoke. The 9.25 on my 2 wheel drive has 100,000+ miles and while I never abused it, it had to tote and tow some really heavy loads on the farm. It's a heavy half ton, but served its time on the farm doing the job of larger trucks and the differential is in great shape. I pulled the cover, cleaned out the goo, found no issues of wear, play or hard surface flaking off the teeth, so I replaced the gasket and filled it with lube.
The parts truck was a full time 4 wheel drive with 360, meaning the rear axle on that truck never had to transmit 100% of the power to the ground as my 318 truck did. The 360 isn't that much more powerful than the 318 of that era and if one axle was going to be bad, I'd expect it to be the 2 wheel drive. I can't testify to the treatment the 4 wheel got, but the frame was sprung from hitting a stump, the rod bearings had chunks of copper torn from them, the crank scored, the cam was flaking hard surface and the cam bearings were failing. I suspect it was doing some steep hill climbing while low on oil, resulting in no lubrication to the bearings. My boy drove it 25 miles to home and frankly I'm surprised it made it.
