Update: I got a new driveshaft the correct length installed a while back (I haven't been on this site for a while) and it turned out that it did not change anything.
Still a bad vibration over 60mph and downright scary beyond 70mph. So I've been driving around the past few years mostly staying off the freeway.
Now: Recently did some suspension work including new manual steering box (old one had excessive play), new torsion bars 1.03, added front sway bar and rear sway bar, rebuilt front lower control arms, 4 new Bilstien shocks, rebuilt rear leafs. Also note that I removed the rear long shackles that I was using as a cheap lift kit and the vehicle pretty much sits level now.
Vibration problem is now fixed!!! I've been up over 80mph without a problem.
Question: What do you think fixed it?