2000 Cadillac Escalade rear drive/ differential

While driving it felt as if I hit a pothole and then my vehicle would not respond pressing the gas. It does rev the engine but the truck does not move. Once I placed it in park, the vehicle continued to roll. The shaft connected to the rear wheels turns but the rear wheels do not move. Is this a differential problem. Should I just replace the "rear"? How much and is it worth it?