Tom, for what it is worth, I had a very similar issue with my '67 Super. I had the drive shaft balanced several times locally and still had the vibration. Finally took it to an old friend across the state that rebuilds drive shafts on commercial trucks. He called me shortly after I left and said "**** thing is bent, you can't balance a bent shaft" He then straightened it, balanced it, and it was perfect after that.
Good luck, Mike
Wish I could find someone around Nebraska to do that. Only two truck places in the state that do it and they don't have the proper hardware to match either end of the two piece shaft.