This doesn't necessarily sound like your problem, but possibly the splines in on the driveline are a little worn, now the yoke has a little bit of play over the spines, this will cause vibration. On mine if I have the gas on it is perfectly smooth but if I let off to coast the yoke will start vibrating... which translates through the floor board, to the seats, to your behind, and the steering wheel....     Its a simple fix, just need to find a good tight spline set, maby off a newer truck with low miles,,,, possibly off the front drive shaft since it would have less wear...