Assuming transfer case OK, what are the most obvious reason why doesn't go into 4WD
Body Control Module
Stuck transfer case motor is the most common, sometimes you can lightly tap on it while someone is pushing the switch or knob, if you hear it run then you have found the problem. Note this must be done on level ground and no load on the drivetrain at all due to the motors weakened brushes it may not have enough power to switch gears if there is any load on them. HTH
Listen to Tennisshoes, he knows his stuff.
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