Could be a couple items. The front actuator is vacuum controlled. When you push the button the 4WD control module sends a signal to a vacuum switch. It moves and sends vacuum to the apply side of the actuator and vents the disengage side. This moves the locking collar over the splines and locks the axle. When you push the button to go into 2WD the 4WD control module releases the vacuum switch and it should move back to the other position. That applies vacuum to the disengage side of the actuator as well as venting the apply side and uncouples the axle.
Common failures are the vacuum solenoid and the vacuum lines themselves. Have also seen times the vent was pinched or had bug nests in it that stopped it from venting. The actuator can fail as well but that usually means it will not shift into 4X4 to begin with.
Images (Click to enlarge)
Sep 8, 2017 at 10:21 PM
