Will not stay in park on 2001 Ford Expedition

Car will go into all positions except park. It will occasionally catch but for the most part will just roll. Must use emergency brake. How to I fix it. Have been parking on a hill, may have something to do with it.

Asked by for the 2001 Ford Expedition
Sounds like the parking pawl either needs adjustment or replacement. This is a 2 to 2 1/2 hour job and is usually done by a transmission some shops to see if they can handle this