rx7jake

im thinking its the driveshaft. i rotated the tires to make sure one didn't throw a weight. changed the rear diff. there were large broken pieces of the ring and pinion on the drain plug. bearings are good. and drive axels seem good. would a bushing cause a vibration at speed? and has anyone ever gotten their driveshaft balanced? everywhere i call says they can't do it the "u" joints can't be replaced.
help would be greatly appreciated.