make sure the linkage is allowing it to go all the way to the detent position. Put the parking brake on, chock the tires, then put it in park. Go under the truck and check the linkage to the tranny. Loosen the bolt that tightens the shift rod and see if the lever on the tranny will go any further towards the park position. If its ok, it may be the parking pawl in the tranny. Im not sure how to replace that, but guaranteed someone here knows, just hang tight and the info will start rolling in.