I am replacing some differential parts tonight after finding out there were some wear issues in my '89 4 series. Everything in them is very simple, with form following function in the most extreme way I've ever seen in anything from Jeeps to aircraft.
If you're looking for drive line problems, even if it has NOTHING back there, for $100, I'd say it's worth it. Moving under it's own power means that there is at least something to work with, though.
But just from what I've seen, take a little flash light with you; look at the inboard top side of both rear wheels, where the axle comes out of the fender. There is a greaseable bushing back there on both sides that can wear all the way through and let the axle start wearing into the fender. When that happens, as I've recently found, it will tweak the chain and sprocket in the diff and allow them to contact the differential housing and wear there. If that's the case, you'll need to replace the worn fender and bushing.
Also, wiggle the steering handle back and forth and look at the steering bushings to see if there is play there. You'll probably want to replace all of those bushings and maybe the tiny little king pins; you won't know about the king pins 'till you pull 'em out and inspect them.
After you see if those bushings still have ANY life left in them (and you'll probably need to replace them...all of them, front and back) flip it up on it's back end. If the seal at the base of the dip stick tube is bad, it'll leak some oil, but dont worry about that...plan on replacing that seal.
When it's on it's back, take a look at the differential, on the right fender, and the chain case, kinda in the middle of the rear axle assembly. Pull the plugs and see if there is any grease in there and if any of the housings have holes that obviously shouldn't be there.
The good news is that EVERYTHING on these is cheap and very easy to replace. And Parts Tree has online diagrams how everything goes back together, as well as incredible customer service when you need to replace things.
