Meaning that each is lined up with the other, instead of being clocked a little wrong.... but you have had 3 different d-shafts in it so I would say that you have eliminated that(still just guessing)
How bad of a viration is it at 80mph and what does that translate to in rpm at the engine... its a rpm issue and not a speed thing. I'm sure that if you changed gears in the rear that the speed of the vibration would change so its all about rpm of any given component
