Senior Advisor at GMC sent me back to the dealer. Shop foreman advised that it may be the leaf springs and because it was out of warranty, would cost me to replace them and that there is no guarantee that it would fix the problem. Since it was just out of warranty, GMC offered to pay a substantial part of the replacement but my part would be just under $300. I'm not going to pay $300 on a "maybe". Of course, if it would have been a driveline problem as I suspect (differential), it would still be under warranty. Waiting on GMC to call me back because they would like an engineer to look at the truck. Will advise of what he tells me.