Need some help tracking down a speed related vibration

[MarkH]

My car has had a vibration since I first got it about a year ago. I have not tracked down the source as I have been more involved in getting some deferred maintenance caught up. The vibration is speed related as it starts up at about 25-30 peaks at around 40, and fades off at about 50-55mph. I can feel it in the steering wheel, but it is much more pronounced in the seat of the pants and feels like it is coming from the back end of the car.

In the last year I have done the following:

Replaced control arms and control arm bushings
Replaced tie rods
Replaced sway bar links and bushings
Replaced rear subframe bushings
Replaced rear trailing arm bushings
Replaced shocks and shock mounts
Rebuilt and rebalanced the driveshaft
Replaced CSB and flex disk
Replaced transmission mounts
Replaced all four wheel bearings
Replaced pads and rotors on all four wheels
Replaced right rear axle, old one was FUBAR
Swapped LSD for open differential

I recently had the wheels rotated and rebalanced and the tire shop did not think the wheels were bent. As just about everything was replaced, I am a little stumped by the source and would appreciate some guidance here. I am leaning toward a wheel or tire problem, and am trying to find another local E3 owner who will let me borrow his wheels for a short drive.

So any one have any ideas on what might be the source of this vibration?

[Jarvis]

Go to a shop with more E30-specific knowledge.

First, ask them to mess with your wheels and tires. That's the easiest thing to check and in most cases, the actual root of problems.
Then have a qualified tech check out the underside of your car. The rear and/or front subgrame may be tweeked. This is a problem in more cars than people admit, and is not easily diagnosed. Although, something like a FUBAR'd front end (as far as I'm experienced with) show's up at highway speeds, not ~40mph.