F-150 rpms plummet with heat/ac?

Late last winter I started having trouble with my heat in my 91 F-150, 4.9l 5spd. When I turn the heat (or A/C I later discovered) to defrost, floor/defrost, or A/C and Max A/C, my rpms drop to about 200, the truck ALMOST dies (occasionally does), then they pick back up to about 1000, drop down, up, down, etc.. Yet when I set it to vent or floor, everything runs fine. Really not sure where to start, as the different blower settings seem to be the cause, I'm not sure if I want to dive into the blower motor to start or what?