While going down the highway at say 70-80mph, I get a vibration that happens when I hold the clutch in. Could the driveshaft be unbalanced? I've had other vibration issues related to the drums/rotors, but those are now fixed. Ialso get a very slight rythmic vibration while cruising. It is very slight, but with the clutch depressed the vibration is definately becomes more pronounced.