While I usually suspect the center bearing carrier *first*.  It doesn't
much matter.  Whenever you replace one of these, it usually makes sense
to replace both.  They're both cheap and easy.  They are also much more
likely culprits than anything having to do with either the U joint or
the half shafts.

BTW, unless the E30 is different from the E28, if it has an automatic
transmission, it will have no guibo.
--
C.R. Krieger
(Been there; done that)

P.S.:   If you're not familiar with the term, 'guibo' is the flexible
rubber disk (also referred to as a 'flex disk') at the rear end of the
transmission that links the driveshaft to the output flange.

I think there is a shaft extension on the transmission that engages a
bushing in the driveshaft to assure proper alignment of the driveshaft
regardless of the condition of the guibo.  I suppose the bushing could be
damaged but I haven't seen that happen.  Psycho, could you try to explain
what you mean by hop type vibration?  At what speeds is it apparent?  Is
there any accompanying noise?
 I had an experience on my 318i where it developed a vibration that seemed
to be at the same frequency as the drive shaft rotation.  It was most
pronounced at about 35 mph where the car 'sounded like a pickup truck with
snow tires on it'.  After removing the drive shaft and finding nothing wrong
I discovered the problem to be a front wheel bearing.  The balls rolling
over the dent in the inner race just happened to be at the same frequency as
the drive shaft and the impulse was resonating with the coil spring
producing quite a loud noise.
Anyway good luck finding it.
Jack

 Well, my suspicions (and all of yours) were correct. The guibo had
basically self destructed on me. Looks like I'll be going back to the
BMW dealership tomorrow to get a guibo ($70) and most likely a center
carrier bearing ($??). Can't see replacing one without the other and I
missed the cutoff for getting it the Bavarian Autosport for delivery
tomorrow...