Worst case scenario you need a new shift cable. Doing the whole cable you'll have to partially disassemble the steering column to replace it, but if all it needs is adjusted the adjustment clip is right by where the cable connects on the side of the transmission. It should just be vehicle in park, loosen the keeper, pull cable housing snug (not too tight) tighten keeper. The "keeper" is most likely just a locking clip. Its been a while since I've pulled a 545RFE so I can't remember for certain.