My 2003, 6.0 diesel, won't allow the A/C to blow air out of the dash vents. It only comes out of the defrost vent or the floor vents. I do have good vacuum and I replaced the selector switch.

Run the Electronic Air Temperature Control (EATC) diagnostics. See if any codes were thrown. Record any codes. Afterwards, erase the codes and see if the problem is gone. It sounds like the blend door actuator may be hanging up.