It sounds drive shaft related! Maybe give this a try to isolate the problem. Put the rear of the car up on jack stands (be safe!) engage the transmission and run the rpm up to where the vibration usually starts. If it doesn't vibrate, I would guess that the problem is in the suspension or body. Some part of the car that would only vibrate if the car was at speed. If it does vibrate, it is probably in the drive line as the body and suspension are at a standstill. Now, disconnect the driveshaft and run the rpm up. If no vibration, the problem is somewhere from the driveshaft to the back of the car(drive shaft balance, u-joints, rear end components,etc). If there is vibration, I would look from the tranny forward somewhere in the drivetrain (tranny mount, motor mount, bad harmonic balancer etc).
I had vibration once that was caused by the u-joints failing. The faster the car went, the worse the vibration got. Almost no vibration occured until over 40 miles an hour. I also gave myself the problem once by not getting the rear u-joint properly seated in the yoke! duh.
I hope this is helpful, as this will drive ya nuts!