If the differential has been serviced and someone put in a limited slip / posi additive, that's a big no-no on the Eaton Gov-Loc unit which is what RPO code G80 is on a truck. The additive is considered a contaminant and can cause the G80 to work improperly. There's a TSB out from GM and this has been debated to death (regardless of official word from GM) but fact is, it's a problem. If you think this has happened, you could try servicing the diff again and be sure the correct fluid is used - I think in the GMT800 trucks like yours, they specify a synthetic gear oil? Check your manual; I don't have late model info.