If you do just replace the center bearing, make sure you mark the driveshaft so the it goes back together at the same place. The drive shaft is balanced as a whole unit. Although I have yet to replace a driveshaft, although it will become a notch in my belt this summer, I would venture to guess that a rear diff swap is probably a little less involved. At least there is less to remove. The diff is right there, you can leave the exhaust in place. The trickiest part is the four bolts that hold the diff to the rear sub-frame. Believe it or not I use a BMW box wrench with one side 19mm and the other 17mm to get to those. It is very thin and is able to get between the body and diff. It is on the short side for the torque required. So I then put my 17mm transmission drain plug tool on my braker bar and put that into the 17mm side of the BMW box wrench. The set-up really works great. I have pulled quite a few diffs. I pulled mine a number of times. Once to swap out my rear sub-frame, once to swap ratios and go LSD, and a few times in junk yards. Just be sure you really tighten the allen bolts that hold the half-shafts to the diff. You don't want them coming loose on you. Be sure they are tight and check again in about a 100 miles afterwards. Jenny Morgon did a column on this once. Does the 533i have a small diff for the E28? The small E28 diff is the big diff on E30s If so it should be perfect for your eta E30! You just have to swap the output flanges and rear cover, but that is pretty easy.