This could be 1 of 2 things.
1. Blend door mount is broke
2. Vac line broke/detached
For number one, you can search on Youtube for my username and heater treater. Video shows what the blend door repair is after you buy the heater Treater part (search for that).
For number two, check your vac lines. An example I recently had a couple weeks ago was after a wheeling trip. I broke my red vac line at the front axle CAD. This caused the symptoms described with the A/C. Runs fine on level, but as soon as load on the engine (hill), defrost and not cold air.
Here is picture after I purchased replacement line and I added an extension. The break was at the union elbow on the CAD, red line.