2002 Excursion Diesel won't stay in park
 

I went to park my 2002 Excursion today and it wouldn't go into park. It sounded like it wanted to, but it went in reverse while making a knocking sound. I turned the engine off and it started rolling backwards on the driveway while making a knocking noise. Could it be something simple like a parking pin, or does it sound like a major overhaul is needed?