Driveshaft vibration will be at a much higer frequency than wheel vibration. By FAR, the most common source of vibration is wheel/tire related. If it does turn out to be driveshaft vibration and the flex discs are solid, then it is possible that someone let the splines come apart during service and did not get it put back together in its original orientation. This throws the driveshaft out of balance.

Good luck,