so you only have the Fan blowing at high speed, there is no Low or Medium speeds ?
As was mentioned, remove the fan, look up through the unit
There is a door up in there, it is called the Recirculation Door.
When the control knobs are set 1 way, Fresh Air enters , if you move the controls, fresh air no longer enters.
You can watch that door move by looking up through that Fan hole.
This time of year, i leave that door closed.
Before you re-install the Fan, look to the left of the normal Fan position
You should see a small motor, , that motor is held on by 2 screws.
Again turn the Control knobs & you should see an arm on that motor move up or down & after it moves, you may hear a clicking noise.
If it clicks & the arm does Not move, there is a good chance, that that particular motor may be bad. You can remove the motor & open it up, then look for broken teeth.
If you remove that motor, the arm i think is still hanging, carefully move that arm, you should feel a slight weight of the door inside the main unit moving, if there is no weight at all, the door may be broken inside.
If you do feel a bit of weight, you should be able to move the motors arm with just a bit of force, you should be able hear the door when it hits its limit points.
Re-install the motor
Reach over to turn your ignition on, do not start the truck.
Now while watching that motor just to left of the fan, you are going to push in the control knob that is closest to the steering column for a period of time to reset the controls
I want to say the button has to be pressed for 10 seconds to reset, but i am just guessing, someone will give a better answer