I had similar problems, changed these items in this order with no help. Shift motor, diff actuator, switch on dash. My son had been driving thru slush and puddles(for awhile). I disconnected the shift motor plug and receptacle, one pin was corroded from water intrusion. I made my own harness from salvaged connectors because they cost WAYYYY too
much ANYWHERE else. Problem fixed, When you slice new harness, make sure 100% it is sealed from the environment. I used commercial aircraft grade environmental splices, works like a champ, just takes some patience. Good luck.....