make sure 4x4 is not partially engaged , seen that before , not trying to be silly by saying that , ive seen manual shift 4x4 partially out of gear causing it to not be fully locked into a range , other than that , trans may have dumped ,its almost impossible to damage park mechanism so thats why it works fine , you said you were pulling in the sand , im wondering if you heard any loud pop or anything , like maybe an axle or something in rear end , put it in 4x4 and see if it pulls