[stomachmonkey]

Most likely a flapper.

They can get out of sync if you start them up with the HVAC running.

Best practice is start car with HVAC Off, wait a second or two then turn on.

That said the motors also burn out.

You can hear the flappers move, get everything going and switch between floor and dash vents.

Listen for flapper moving.

If it's the center dash vent you can pull the panel above the tranny tunnel and see it actuate.

104k on the clock is nothing for that motor.

FWIW, don't let the family slam the doors, my most common maintenance item on both my Tahoe and Yukon are window actuators. The plastic take up reels break. Cheap and easy to replace though, just annoying.