I had a similar problem on a Corvette a few years ago. Turned out to be a defective brake rotor.
For some reason, a rear rotor (replaced by the previous owner) turned out to be out of round causing the vibration.
After I replaced all U joints, both half shaft, dive shafts without any positive result, I found the cause of the problem by accident.
When I needed a brake job, the mechanic found the the rotor was totally out of round on his disk turning machine" replaced the rotor and everthing came out smooth . A long shot, but it worked for me.
jackD