I have a vibration inside my car that seems to be coming from the drivetrain , its not real bad but you can feel it in the seat and the shifter. It seems also to be worse on county roads and better on state highways, at 60+ mph. I jacket the car up and put it on jackstands and started the engine reving it up to abour 3000 rpm. it didnt seem to vibrate I then put the car in gear ,brought the speedometer up to 60+ mph and the cab,seat and shifter have a definite harmonic vibration that is annoying the heck out of me. the tires have been balanced but I dont know if the driveshaft has or needs to be. the transmission is a C4 and the rear end is an 8.8 ford. The driveshaft was proably a custom because the car is a 34 kit car I bought half finished from a gentelman who ran out of money.