ok i ran my car on the lift to 70mph where it vibrates badly, a guy was looking underneath and said the driveshaft was fine (2nd one in the car) i started getting this when i changed the flywheel/clutch/and rebuilt the transmission, ok so the vibration sounds like a vroom vrooom vrroom it will do it in 4th or 5th and is strongest at about 75-85 then diminishes slightly. what do you guys think it could be? when i press the clutch the wicked vibration goes away.

 

Sounds drive shaft related. Have your mechanic check drive line angles. The tail shaft of the trans should be up 1-2 degrees higher than the plane of the drive shaft & the rear pinion shold be 1-2 degrees down from the plane of the drive shaft. I just went through this problem myself after a rear & torque arm swap.