Cant get enough airflow to defrost windshield fans blowing max

Suspect a defective blend door actuator motor that controls airflow for the defrost. This electric motor may be defective and in some makes and models can be extremely difficult/expensive laborwise to replace

How long have you been driving or owned a car? Your A/C is set on front "defrost" instead of main. Turn off the defroster for the front and select the switch to read "main" or if your A/C panel uses icons to display, press the one where it looks like the airflow is blowing air (represented by arrows) towards you and not the windshield. The arrows on your buttons or electronic display refer to which way the air is flowing. Choose the one you want where the arrows show the airflow is blowing out towards you and/or turn off your defroster. (If you have a knob to select airflow, turn it so it is no longer on "defrost" and it points to the main airflow blowing outward).

. Air won't blow out of the floor or center vents just defrost ? Do you have manual or automatic climate control ?
Air Flow Control
Air flow control is accomplished in the following manner:

• Primary control is through the function selector knob mounted on the A/C-heater function selector switch, which is part of the integrated control panel.
• The function selector knob has the following positions: OFF, MAX A/C, A/C, PANEL, PANEL/FLOOR, FLOOR, FLOOR/DEFROST and DEFROST. The A/C-heater function selector switch combines a vacuum selector valve with an internal electrical switch.
• The vacuum selector valve directs source vacuum to various vacuum control motors (18A318). Refer to the Vacuum Control Diagram , System Airflow Schematics and Vacuum Control Chart .
• Two internal, single-pole electrical switches are also controlled by the A/C-heater function selector switch. The combination of these electrical switches controls the electrical supply to the A/C clutch and heater blower motor switch (18578).
• The position of the function selector knob determines the manner in which the system will operate.
• Each position of the function selector knob is detented for positive engagement.
• Vacuum Control Motors The vacuum control motors (18A318):
  
  • are located on the A/C evaporator housing, A/C recirculation air duct and heater air plenum chamber (18471).
  • direct system airflow to the vehicle interior as determined by the remote climate control module or manual override.
  • The five-passenger seating configuration has an additional vacuum control motor mounted on the heater air plenum chamber to control airflow to the rear seat A/C register (19893).
  
  

Usually this is a problem with the blend doors under the dash. You either have a vac leak, a dead motor, or a faulty door.
The control opens and closes the doors to direct air thru the heater core and evaporator core along with directing air thru the vents.
When a problem occurs, the default position is the defrost vents to make sure the windshield remains clear.
You can do an internet search for "repair durango blend door" for more info.

It appears you know why your ac starts to blow warm air...not enough airflow thru the condenser. A aux fan would definatly fix this but two things you can try. Take compressed air and blow out the condenser but not from the front side... blow from the engine side. Small dust particles will collect inside the condenser over the years and pack in restricting airflow. Hopefully this takes care of it.
The other thing is to have your fan clutch checked it may be acting up on you which will also not pull enough airflow thru the condenser. If all else fails say hello to a electric cooling fan. Good luck!

No airflow at all or just not cold? The AC only runs in the AC , Max or either of the defrost modes to automatically dehumidify the air to reduce fogging. The other positions vent in outside (non conditioned) air.

sounds like a blowermotor to be sure a easy check is to use a testlight and hook clip to blowermoter ground wire and backprobe powerwire gound will be black power purple or orange. if it lights with speed on high motor bad u can tap with hammer or back of screwdriver see if it runs.

clean or replace the cabin air filter. It is located behind the glove box. See owner's manual for directions.

The Climate control system in your truck has a gate that changes postition in order to change the flow of air from your face to defrost. The door is actuated by vacuum. Yours is still working otherwise it would be on defrost all the time. So you do have partial vacuum to the actuator but not enough to keep it in position when on high. Sounds like you have a vacuum leak but not bad enough (yet) to totally disable the gate.