In my case of this symptom, it was the passenger rear CV shaft. It would go right crazy under any load in fifth on the highway.. feeling almost undriveable. I thought for sure it was the driveshaft and replaced all the carrier bearings and the u-joints. I finally bought some new axles for the rear from a junkyard and fixed it. I have since found that bad carrier bearings and U-joints in the driveshaft manifest themsleves as a "rubber band" effect when taking off in low gears, or driveshaft thump under hard acceleration in first.