I have an 89 GMC TopKick that is a line truck, single axle. There is a differential lock light and a buzzer that is on and will not go off. I don't have any manuals for the truck part of my rig. Can anyone tell me how to get the light/buzzer to go off or how to unlock the differential. I've looked everywhere and can't find a button, knob, or switch that has anything to do with this issue.