Try to take the linkage off the shifter arm on the side of the tranny and see how stiff it is to get out of the park position by hand. Should be stiff but doable with plier leverage. If this checks out, with linkage still disconnected, move the shifter thru the pattern, if it will move. With an assistant, observe linkage articulation from under the truck. There are a bunch of bellcranks and shafts and brackets and shields jand pivots and bushings on these things, and the bushings require lubing. My bet if the tranny lever moves ok is lack of lube at a disadvantageous angle. If the tranny lever is the problem, you have internal parking pawl problems of some kind, and parts may be getting ready to come loose inside the box. Not a good thing.