Sounds like your diff. has perhaps seen better days. Does the diff. make a howling noise when you're just coasting along w/o accelerating or decelerating? If so, chances are the bearings and other internal diff. parts are worn beyond spec.. The slop in the diff. due to worn parts can cause the diff. carrier to sort of float around slightly in the housing when you're just cruising along down the highway. IOW, the slop in the diff. becomes most noticeable without much load placed on it since there's no heavy force applied to take up the slack and noise occurs as a result. Check the gaer lube in the diff.. I have a hunch that it probably looks and smells pretty nasty!