Hello I will help you with your question,
There is a temperature door motor just above the hump in the floor that controls the drivers temp. In this controller are plasticx gears that will strip and leave you with uneven temp from side to side. If you replace the temp door motor you should have restored function.
This is a view from the bottom showing the motor, you will need the number off the motor to get the correct replacement. Disconnect the electrical connector, remove the two 5.5 mm headed screws and the motor will pull free. Install the new motor and turn the truck on without touching the controls the system will cycle for a minute until all the door positions are learned and it should operate properly from there.
Let me know if you have further questions about this