The rear differential in these is a limited slip typically with friction modifiers and molly lubrication additives. Did they put the right amount in? Did they change the universal joints in the rear joint when the diff was changed? I'd take it back to them for a road test. Also confirm it is the diff and not the wheels. Over/under inflation can cause noises you may be relating to this but infact not have anything to do with the replacement. But again have a good mechanic (hard to find these days at dealerships) do a ROAD TEST with you in the vehicle and hope it makes the noise...try to also determine the conditions when this happens...temperature, time of day how long the vehicle was running etc....document this and again a good mechanic should be able to source your new found noises. Good Luck!