Hmmm. I'm assuming you have the torque tube setup, yes? And this wasn't happening prior to changing the U-joint? Sorry if I've missed something here, Dave. Just trying to unscramble the egg a bit more. Thinking about this while shaving this morning, I did many years ago have a somewhat similar situation and it turned out to be the muffler (which was going bad). At some speeds it would produce odd vibration noises which would disappear once I slowed down. But this seems to have begun when the U-joint was changed, so I doubt it is something like that...although if you move the exhaust pipe I've seen those start to make odd noises sometimes...rubbing on the manifold. Another long shot.
