Just out of curiosity... could it be that both rear tires are missing 1 lug nut each? I checked the driveshaft and tranny mounts and seem to be in working order... rear was sealed and overfilled with fluid. Its kinda mysterious to me but I dont have 2 extra lugnuts to check... :-/
If I had two jackstands handy, I would videotape it accelerating with the rear off the ground... technically the car isnt registered or on my insurance yet, and I dont want to take another chance driving it.
And JR... it basically sounds like... imagine putting a credit card on a pedal bike just enough to touch the spokes... except not as fast and a little louder