Funny thing I also did some maintenance to the rear diff today and yesterday on my friends car, but an oilchange wasn't enough (he had already tried that).
The diff was making alot of noice and he could get the car to reverse.

Yesterday we removed the rear differential and gearbox (they are bolted together in the rear on this model):



Here is the problem, a hole where you can see the crownwheel:

We haven't opened the diff yet but we could here that some metal ballbearings was rattling around in there.

So today we went to the junkyard and removed the whole rearaxle from a simillar car:


We hope to have his car running again this weekend or the next week.