Al of the airflow comes out of the defrost vents.

Check for vacuum leaks under the hood to inside the truck.

There are mode doors that can get jammed with debris. They need to be checked.

Roy

Edford, can you hear anything moving in the airbox under the dash when you change from heater to vent to AC to defrost? Also check to make sure all cables, wires, and vacuum connectors are properly connected.