While checking the driveshaft, make sure all the U-joints are in the same phase (the cross between the yokes should point in the same direction as the rest of them on the driveline). If the shaft was ever off for some kind of maintenance it is possible the slip yoke was put back on while rotated a tooth or two over on the splines getting the joints out of phase. This can present as noise and vibration at higher speeds but seem to have little or none at slow speed.
