Air cond door vacuum

vacuum leak or bad check valve I would suspect.

that's my guess *grin*

You have a heavy vehicle with an anemic motor. Anything operated by vacuum will eventually fail once manifold vacuum goes to zero (wide open throttle). This can be delayed by using a vacuum tank fed by a check valve. This was common practice in the fifties with the vacuum operated windshield wipers in use then. Of course any vacuum leak exasperates the problem. Steve

If you can hear a flap or something shutting in the vent system then it is probably a check-valve of some sort or a loose vacum line. I had a similar issue on my 99' ram 1500 4x4. Every time I acclerated hard the dash vent would shut down until I eased up on the throttle and the front axles for the 4-wheel drive wouldn't engage. My problem was a vaccum line that connected the auto locking 4-wheel drive and the a/c vents was disconnected.