There's no cabin air filter on these trucks. Not even on 4th Gens... So dust will collect on a cold and condensated evaporator (the thing that gets cold). Because of this, the dust collects and will clog the evaporator over time. That mud film it leaves can fall off and clog the drain. You can either tear out the dash to take off the EVAP assembly and clean/replace stuff, or... You can take out the blower motor and resistor block, and blow it out with compressed air and water.
I do mine with an assortment of toilet bowl cleaners I can get in there. As well as compressed air, and a weed killed sprayer that I fill with water. Then I bend the head to my liking. It's a *****, but it takes about 30min and works 95% as well as taking the dash apart.
Good luck!