indexing
before you do anything else,
mark the location of the driveshaft the way it sits right now.... mark a scribe in the driveshaft itself then a mark on the pinion yoke (what the driveshaft bolts to) make sure there is no play in the driveshaft/ transmission outputshaft/ pinion bearing. do this by tugging pushing pulling yanking. whatever you have to do to find any type of freeplay in the components. if all that is good then move onto this step:
make sure the pinion yoke is not bent. unbolt the driveshaft. then proceed to rotate the driveshaft 180 degrees in relation to the pinion yoke. torque to spec. retest the vibration.
this is sometimes the problem....if the vibration still persists. a driveshaft balance weight may have fallen off.
in this case, return the driveshaft to its former location and have a rearend shop balance it out.