Drivetrain Vibrations?

[JR'Z 525]

How do driveshaft problems, such as worn flex disc or center bearing begin to manifest themselves? I've been getting what seems to be a progressivley worstening vibration for the last several months from the rear that begins at 53mph and really never ceases. At first I thought it was worn out DunlopA2's, In Jan I got new Dunlops then a month later new Rondell 58's. Even with the old e38 rims and new tires I had a problem. Same when the Rondells were installed. The R 58's use a plastic centering ring which I don't think is very durable so I got a set of metal ones from the Tirerack. These did not work well at all (fit was too loose) so back to the snug fitting plastic ones. The Roadforce balance also condemed the LF and RR tires when the Rondells were installed. The local tire dealer got 4 more A2's and swappped the 2 bad tires out. Still getting vibration. I find it hard to believe out of 6 tires I might have another bad one. I think with 156,000 miles maybe I need to take a close look at the drive shaft components. Since the exhaust system and shields share the ds tunnel, what can I look at if I put the car on a rack? Any other ideas?
Help...this car is not driving like a Bimmer should!

Thanks,
JR :(

[JR'Z 525]

To add to this, I judge the vibration by: 1. how bad I feel it in my butt, 2. how mush the passenger seat & armrest vibrate, 3. and how bad the console mounted cell phone & cradle vibrate, 4. sunvisor unclipped. The sunvisor doesn't vibrate near as bad as the seat does. I guess that's a testimony for BMW body/suspension isolation mounts.
JR