I have never looked into driveshaft issues on my E28, but I just got through a bit of work on my CS. I think the drivshaft layout is similar for all old BMW's.

The center bearing failed (the rubber tore) and it turned out to be caused by a sagging rear tranny mount. This caused the center bearing to be pushed down.

I bring this up because if it's not a balance or u-joint issue, then it's probably an alignment problem. One way to check this is to remove the nuts from the center bearing and see where it wants to sit naturally. It should be pretty close to where it bolts-up. The guibo is fairly stiff and capable of supporting the front driveshaft.

I also put a length of fuel hose in the center bearing to make it stiffer. I didn't want to fail another center bearing incase I hadn't soved the real problem.

How did the old guibo and center bearing look?

Devinder