Help Confirming Source of Rear-End Vibration

[Eric]

Hi all.

My '97 ti (replacement for the 98 I drove for 10 years and totalled a couple of months ago) has developed a powerful rear-end vibration.

Details: 207,000 miles, Currently running 15" snow tires on steel rims, M-tech springs & sway bars, Bilstein shocks.
Brakes good. Replaced McPherson bearings last month. PO replaced R outer tie rod end.
In looking for this problem, I discovered that both of my inner tie rod ends are shot, but only barely (I'd checked them when I first got the car and there was no play, now there's barely perceptible play - lesson: always replace inner and outer together, like the PO did not do).
I have rotated both rear wheels with the pads pried back to reduce drag, and rear wheel bearings feel smooth and have no play that I can feel, fronts seem good, too.
I have had both rear wheels rebalanced, and have also swapped them with the fronts, all with no effect.
I've crawled all around under the car and the universals and CV joints look and feel good, and the axles and driveshaft do not appear bent.

The problem: A harsh vibration that feels like it's coming from the rear of the car (occasionally left, occasionally right, usually middle), that I can feel mostly in my butt, like a back massager gone haywire.
No vibration in the steering wheel.
No vibration in the gearshift (MT).
The vibration is affected by throttle position and road speed - it's worse at steady throttle, better with acceleration or deceleration, worst at about 40mph and again at about 70mph (can be bone-jarring when it's just right), and smooths out pretty well at about 80-85.
It is not affected by engine RPM.
There is no sound associated with it - no howling, growling, or grinding.
Taking sweeping turns in either direction does not change it.

These two threads, and a few other things I've read, lead me to suspect the U-joints / driveshaft:
http://www.318ti.org/forum/showthrea...ight=vibration
http://www.318ti.org/forum/showthrea...ight=vibration

I know that the U-joints are not worth trying to change, and that the driveshaft ain't cheap, so I'd like to have a pretty good idea of the problem before I throw parts at it.

Can anyone say anything definitive about this?

Thanks,

- Eric