I found SOMETING. First, you have to removed the rear diff and driveshaft mount...some say they have been able to remove just the rear input diff/driveshaft mounts with the exhaust in place, but I highly doubt it. You'll have to remove the exhaust before the driveshaft can be unbolted. After the exhaust is removed, you might then be able to move the d/s out of the way. After that, the input seal is a pig to work on, apparently.
I've heard some have been able to treat it like an output seal, but by looking at pictures of how the input and output seals are assembled, it looks different enough to be nothing like an output seal.
You have to removed the d/s mount on the diff side and you have to remove all the other parts inside (obviously) with some sort of puller...
The part number I believe you want is this: 33 12 1 213 949
If you haven't seen it already, here are the inner parts that the diff's input has inside...just so you know what you're looking at.
http://bmwfans.info/original/E32/Lim...3/ill-33_0073/ (keep in mind, this link is for my 02/1990 production 750iL...it may differ from yours)
The actuall removal I'm not too sure with. It's odd that there wouldn't be a write up of some sort. I have to look at the input on my diff in a few days (not the seal) and I can take some time and pics on how to remove it if you like?