[gmtech79]

Year would be helpful, but yes does sound like a vacuum problem. Does air then blow from the defrost vent when under load? Find the vacuum reservoir and make sure it is connected to vacuum and doesn't leak. Under load an engine has none or less vacuum than when at idle or steady state cruise. It has less vacuum since the throttle is open (or wide open) and manifold pressure is closer to atmospheric pressure. Think, whenever the throttle is closed and you have a cylinder on the intake stroke, (intake valve open, piston going down drawing in air/fuel.) the throttle blade acts like a restriction to atmospheric pressure and the piston creates a "void" or negative pressure in the intake manifold. With the throttle open you have less restriction and equalize the pressure inside and outside of the engine hence less vacuum. That is why on a hill under load and have a leaking vacuum reservoir you lose that negative pressure and the hvac defaults ( goes to defrost or the normal position)
Sorry for the long post and hope I didn't cause more confusion than good.