There is also a high or low gearing selector on the tranny. You must manually switch between them. You have 2 low speed gears for climbing or 2 high speed gears for racing. But you do not have all 4 at the same time.
You can't shift while the truck is in motion you have to set the truck up for either high or low before driving, which is really easy to do all you have to do is adjust the knob selector.