Took my Blazer out today and had the air on Max ac and full blast as its over 105 today, as I got up to 70 MPH I thought it just quit then I realized that it was blowing out of the defrost instead of the vents, I turn it to reg AC and it came back through the vents and worked fine the rest of the way home. not sure what would cause it???

97 Blazer 4.3 lt 4X4 automatic trans

 

Sounds like a vacuum issue. Max AC recirculates the air in the cabin & regular AC cools outside air being brought into the cabin. Seems the blend door wasn't operating properly & that could be a vacuum issue or blend door actuator.