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driveshaft question (kinda longish)

Hi all.  Well, I was going to get new tires this weekend, but as fate would
have it something else is wrong w/ my car.  Shocker huh.  O.K., here's the
problem- I had the center support bearing replaced about three months ago
(the BMW part conversion).  This included checking the driveshaft out,
servicing and re-packing the drive universal and replacing the very worn
center support bearing.  Prior to this service the car had a slight steering
wheel and dash shake at a very small speed rage- about 70-75 mph and usually
under throttle.  I assumed it was an out of balance tire or ball joint
etc...  I had the front tires balanced and the whole front end checked out
and all was good and in spec's.  I then concluded that the shake was
probably due to the play in the worn center support- which a knowledgeable
mechanic said was very plausible.   After some time the bearing started to
deteriorate very quickly, so I had it replaced.  Needles to say the slight
shimmy didn't go away nor did it get any worse for that matter.  I was
somewhat perplexed at this but because it was very minor, it didn't warrant
immediate attention.
    I did notice however, that the shake got much worse- wheel and dash
shaking became more severe and the speed range became wider- about 65-85
mph, when the temperature warmed up above sixty degrees.  When it got cold
again, the symptoms returned to their minimal characteristics.  This lead me
to consider rubber mounts- either sub-frame, tranny or A-frame mounts that
would be affected by temperature.  Well, in the past few weeks the shaking
has gotten very severe at certain speeds.  It is fine up to 65 mph and
begins to shake from there to about 85 mph.  When I say severe I mean the
whole car shakes- from the dash to the hood to the seats etc...  It's bad.
    So I took the car to my mechanic who said it was probably bad/worn
sub-frame/transmission mounts.  He replaced those @ $140 but it had no
effect on the problem.  He put the car on a lift and  had someone drive and
accelerate through the gears and he notified the driveshaft started to
wobble at certain rpm's.  He said this is what is causing the shaking and
the driveshaft needs to be taken out and sent off to be balanced.  At this
point I'm about ready to lose it.  Anyway, I was just wondering if this
balance procedure is the solution or if it could be the center support
conversion that is causing the problem- bad part, not done correctly, etc...
It still is affected by temperature which leads me to think that it could be
something else.  The guy who did the conversion said he matched the front
and back when doing the bearing and he has done many of these and said he's
never seen one come out of balance after the installation of the bearing.  I
would very much appreciate any suggestions or input about similar
experiences with driveshaft problems.  Sorry about the length.

Thomas C.  Turse
85 4ksq
Evergreen, CO