it is probably the vaccume acctuator under the battery tray. first you should check the t-case motor. turn the key on but dont start the truck, then push the 4hi button and listen for the sound of an electric motor followed by one click. if you hear that the shift motor is most likely good. after that disconnect your battery and remove battery tray under the tray you will see a metal and rubber plunger looking device. disconnect the metal cable from the accuator and pull out on cable, where the metal cable goes into plastic put a pair of vise-grips to hold out on the cable .reassemble and give it a try.