Blower fan only works on high setting

That sounds like you have the manual HVAC unit and controls for fan and temperature not the type where you just set a temperature and it does the rest. In that case the most likely culprit would be the fan speed resistor pack. It is easy to get to to change. Empty the glove box, now on each side squeeze the plastic in so the two stop tabs come free of the opening. Look in and up at the top of the blower case. The resistor is held in with two screws. Unplug it and swap it out. Because of the way your truck operates by routing the power through the resistor and back to the control module instead of just through the resistor pack, this should repair it, however it may not if the failure is internal to the module itself.