Also to along with what sthemi said, at some point on those trucks, (can't remember if it's early years or later years), but some of them have a short pigtail harness that went from the chassis to the speed sensor in the rear that would get corrosion in it and set the light off. Check to see if you have this jumper harness, because if you do, I would be inclined to replace this first over the speed sensor.