I had a similar vibration...always about 50 mph....it didn't shake the van like
you describe, but it was obviously not normal.  After ruling out drive line
components, etc., the only rtemaining culprit was the tranny....and that's what
it was.  The van still has some harmonic resonance at about the same speed...but
the rebuilt tranny brought it down to acceptable "normal(?)" levels.  Wish i'd
had my flywheel balanced at the time but my ignorance of this prevented me from
asking and it was not offered by the shop as an option.
steve