Check the wire loom going into the motor servo.
I have seen bad wires there cause the motor to lose power.
I believe was the white wire
Speak of the devil, now Im having this issue with it. It stopped going into 4wd again and I wiggled the plug where it goes into the servo motor and it engaged. Is the issue normally a broken wire at the plug or the plug/connector itself? I havent had time to mess with it to check.