i have a 2006 vortec max 1500 4 X 4. while im driving the service 4wd will display. will not go into 4wd, but if i stop and turn off truck. start truck back up, then it will go into any 4wd mode. but again after driving a while will all repeat again. if i could please get some help. wondering if it might be a bad relay or somethin. thanks..