My 2005 Chevy Impala was stuck in Park and would not back up. I was on an incline.

 

It happens. the park pall is stuck because the car roled after your shifted to park and released the service brake. Next time, apply the ebrake to hold the vehicle from rolling, before you release the service brake. Then the parking pawl won't have the weight of the car on it and can move freely.

 

Thank you very much. I will certainly do that the next time I am on an incline.