I've heard of this loads, and I definitely have it! My E34 feels like the wheels are out of balance, but all 4 were just done. It is worst around 55-60 MPH, but is very dependant on the road surface, whether you're going straight etc. Sometimes it feels quite bad others it's almost unnoticable. It is sometimes felt at as low as 30 MPH.

What's the most likely culprit? I'm suspecting some kind of suspension bushes probably at the rear, as the vibration is through the whole car rather than the steering wheel. There are no knocks or rattles from the suspension though.

Cheers
Paul.