As johnc suggested, search the Driveline forum for driveline vibration/driveline angles.
In addition, note that there can be other sources of vibration that mimic driveline angle vibration.
What happens if you are parked in neutral and rev the engine high? Any vibration? Try it in neutral with the clutch in and then out. Just curious.
Another source of vibration are driveline parts touching the body. For example, when first built, my 240Z with SBC and T5 trans the whole car would vibrate at high RPM while driving. The vibration always started just before the shift point and got worse as RPM increased. I suspected that my driveline angles were off. Upon inspection, I discovered that the T5 Hurst shifter housing was touching the trans tunnel under load transfering engine and driveline vibration to the body. Put the car up on a lift and look for witness marks where driveline components might move and touch the body or other parts. Use a crow bar and move components around to see if they touch/rub other parts. I had to trim off about 1/4 inch of the shifter hole on the driver's side to eliminate the vibration.
Since you didn't build the car, a complete inspection of driveline angles, engine/trans mounts, half-shafts, U-joints, suspension etc would be a good place to start.