Drive line Vibrations?
Too much Grease in drive shaft (it wont collapse) when suspension is compressed
Too much slop in pinion flange, make sure nut is staked and tight.
Check to see if your drive shafts are "out of phase" IT should be a straight line from the bearing cap on one end to the bearing cap on the other end of the shaft
Your u-joints are bad or without grease. Replace them.
Your shaft is not balanced Get it balanced at a driveline shop.
Really bad vibrations can be caused by the transmission output shaft in a stock 3 and 4 speed this will be VERY noticeable