You have to backtrack to how the truck was before the vibration and go from there. If it were me I would be running through the ranges on the t case to see what is or isn't happening. If that was fine (and it didn't sound fine in your responses) then I would drop the front and back 2" if you can and see if that helps. But I would probably first pull my front drive shaft and go around the block. Then I would pull the rear and go around (if your truck will move with only a front shaft) . If its a drive shaft recheck phaseing and ujoints. If nothing have the shaft rebalanced. If none of that works then start dropping the truck.