If the truck is an auto- leave the trans in park, turn the key on with the engine off, turn the **** to 4hi, get under the truck and tap the transfer case motor with a hammer and see if it switches. These motors bind up all the time. If it's a stick shift, put it in neutral with the parking brake on, turn the key to run with the engine off, turn the **** and tap the transfer case motor. If this works, keep working the motor between all the settings and you might get lucky.