This is a long shot but the front bshing in the torque tube could have gone oval shaped. I had vibration at 45 mph and didn't know why. For another reason I put an Okie bushing/seal in the TT and the vibration went away. My front bushing which I had to pull out to install the Okie assy was worn oval shaped. As I say it is a long shot and I hope you find out what it is before you pull the TT apart.
