Can't say without knowing if it's an automatic or manual transmission...
O'Reilly Auto Parts
Are you not using the parking brake? You may have a broken parking pawl in
the transmission...from not using the parking brake on hills. ;-)
All of the cars left for repair at our service center are parked in our
slightly-sloping front lot. When we get in to move them into the shop, very
few have the parking brake set. Make sure your park brake is working, and
use it. The parking pawl is a small steel hook a couple of inches long that
is forced to bear the entire 2-1/2 tons of the weight of your SUV if not
assisted by the park brake!
Chino Autotech Inc.,