I guess I have ask you to do some diagnosis. the axle is vacuum shifted but its controled by a switch on the T case that shifts the axle fork. I have seen many that were nothing more then a bad vacuum line from the intake. if its a diesel then make sure you have vacuum. all have to have vacuum to the T case "confirm" it then shift it to 4x4 and crawl under it, should have vacuum at the axle(use a gauge) .if your not seeing vacuum from the t case to the axle replace the switch at the T case. easy stuff have fun.