Air blend doors do not work. Air blows from defroster vents

There are several possible reasons why your blower, vent, defrost, AC, and heater might stop blowing air:

1. Blower Motor Failure: The blower motor is responsible for blowing the air through the vents. If the blower motor fails, you will not get any airflow from the vents.
2. Blower Motor Resistor Failure: The blower motor resistor controls the speed of the blower motor. If the resistor fails, the blower motor may stop working or only work at one speed.
3. Fuse or Relay Failure: A blown fuse or faulty relay in the blower motor circuit can prevent the blower motor from working.
4. Control Module Failure: The HVAC (heating, ventilation, and air conditioning) control module is responsible for controlling the blower motor and other components. If the control module fails, it can prevent the blower motor from working.
5. Vacuum Leak: If your vehicle uses a vacuum system to control the vents, a vacuum leak can prevent the system from working properly.
6. Broken Blend Door: The blend door controls the direction of the airflow. If the blend door breaks or gets stuck, the air may not flow through the correct vents.
7. Wiring or Connector Issues: Faulty wiring or connectors in the blower motor circuit can prevent the blower motor from working.

Vacuum controls this circuit and leaks will cause blend air door to default to defrost. Check for bad check valve or hose leaks under hood first as these are the easy ones to get at and under hood heat causes deterioration.

Usually a sign of a faulty blend door under the dash.
The blend doors direct air thru the heater core and the desired vents.

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.

This is a common problem on this site.
In your case I would look for a vac leak or a sticky blend door.
Since the direction of air flow changes when you are accelerating, I'm going to guess the blend door or doors are controlled by engine vacuum. There may be a vac leak between the dash and the engine, or the blend door may not be able to move back and forth correctly.
Defrost is the default position when a problem occurs, to make sure the windshield is clear of fog or frost.

Often a sign of a blend door problem. The blend door or doors are in the heater a/c box behind the dash.

Common question on this site. On late model cars, the vents are controlled by blend door motors. Small electric motors open and close doors to direct air flow.
Chances are you have a blend door that is stuck or a failed motor.
The default position is the defrost to make sure the windshield is always clear.
Someone will need to look behind the dash to see if all of the doors will move on demand.

There is a blend door acuator which moves when u change your settings. Its a electric motor. Down under dash, Will need a mech and a heating and cooling diagram to find where the motor is,

The most common cause is a bad blend door actuator which controls air flow throughout the cabin. When air flow only flows from 1 section (either floor, vent, or defrost) it's an indication that the blend door actuator is either stuck or broken.

Your vehicle uses electronically controlled blend doors to direct th air flow from defrost to vent and to the floor . If the door motor is bad then it will stay at a fixed position so you will not get the air to flow out the defrost ducts. There are at least two of these one for air blend and one for fresh air. They are located on the passenger side usually under the plastic cover and have a wire harness that plug in and are usually shaped like a piece of pie.