blend door controls hot or cold, that may be a problem, but I don't think so on your truck. Is the fan speed appropriate for which setting it is on (high, med, low)? You will have to judge it by sound since you aren't getting the flow. If the fan doesn't blow at high medium and low speeds, it would point to an electrical problem (blower motor resistor, but usually when these fail you get only high speed, no low or medium). If the fan speeds are appropriate, you may have a mode door problem. The mode door selects which duct work the air blows through (defrost/floor, face, face/floor, floor etc.) If there is something wrong with the door or the actuator, the air will not be flowing 100% where you select and you will get the "weak" flow because you are losing air to other vents.
Check out these videos and they may help:
Heater Treater Diagnostic and repair videos, part 1,2,and 3