If the parking pawl won't engage and hold the car in place then you have a out of adjustment shift cable or the cable isn't allowing the arm to move it into the fully locked parking pawl position for one reason or another. The only other possibility is something inside the transmission has failed.
April, 16, 2011 AT 2:46 PM
Your first assessment sounds correct as I have had no problem with the transmission so far. Thank you for the answer.