Since it changes when you push the clutch in I would doubt it is the driveshaft. Many years ago I had a similar situation and I found that the pilot bushing had backed out of the crankshaft so the input shaft and clutch disc where not being held on center, making the assembly out of balance.


Wes 73 Barracuda 440 EFI 4 speed, 144.85 MPH LTA A/FSS Record Holder.