E-350 A/C switching to heat when accelerating

Ford E-350 5L 80000+ mi.

When driving my van in the mountains last weekend, the A/C would switch over to heat when I was climbing. Fan blowing on the floor and full heat for the temp. It would then switch back to A/C when the engine was not loaded. This was true for both the front and rear units. I am assuming this is a vacuum issue of some sort. Can you tell me how to get started in fixing this?

Thanks,
Paul