Quote Originally Posted by m falconstien
I experienced the same type of vibration and was able to pin point the problem to the flex plate. It seams that some flex plates have a weight for balance on some 5.7 and 4.3's. Ive never had a drive shaft balanced as long as the ends were phased.
i agree with the drive shaft statement, but not the flex plate. if it was the flex plate then it should do it sitting still also.