new 8.25 shudder in low speed turn

Well, the chatter is back--mild when cold but now really bad when warm. It started after I had a new tailpipe and muffler installed--I assume the exhaust work didn't cause the chatter, but the trip to the exhaust shop was the first extended highway driving after rebuilding the rear end. What now? Should I flush the oil again? Try a different lube? By the way, this is a cone-type differential and the cones look pretty good.