2003 excursion auto won't go in gear sometimes after parked

[mike70gtx]

Just got an 03 excursion with 7.3 and auto. It shifts out great and goes right into gear. On 3 different occasions after I parked and shut it down it would not go in gear. I'd go back and forth from park to drive and it would then shift into reverse. I'm assuming it works off of vacuum. What could be the problem? It is a 4x4.