2005 Cadillac CTS Car Moves When In Park

My car was previously in an accident from a head on collission with the previous owner. I bought the car and had it repaired and rebuilt. Everything works fine, it drives fine. However, the only problem I have is that when the car is parked on a slight incline or decline such as a small hill, it does remain still. The car either moves backward or forward depending on how its tilted. It is not able to hold still while in park on an incline but there are no issues on a level surface. Do you know what the problem may be and how needs to be done to correct it?