vibration of drive line

I own s/n 10193, 330GT 2+2.
when I drive up to say 100mph and let off the gas pedal a strong virbration comes from the drive line that is velocity oriented, not engine rpm oriented. I have a new "u" joint and bisket and I am wondering if I have an engine mount problem or some other kind of problem that when the torque is reversed it throws the whole thing out of whack. After a while is it reduced and when you drive beloe 50 mph it again seems fine. Any ideas?