Are you sure you have the 3650 trans? Depending on your car's build date, you could have the T-45. Check your door tag. If the number under the TR field starts with a K, then you do have the 3650. If it starts with a 6, then you have the T-45.
If you do infact have the 3650 trans, it could be that the shift stops on the shifter are not set correctly. Actually, the 3650 trans doesn't require the use of shift stops on the shifter as it has built in internal stops. Most people just completely remove the stops altogether.