Braintrust,
Yesterday, I'm driving a 2011 Ford box truck, loaded to capacity. AC is on and working fine all across the board. When the engine starts to struggle (going up a long hill, flooring it to pass, etc), the air stops blowing from the vents and instead comes out of the defrost vents. I'm pretty sure that it was always Defrost Low regardless of what fan speed the AC was on. The air felt cold from the defrost vents but it was hard to say since it wasn't really hot out. I was solo and it's quite a reach over the dash, especially at highway speeds, so I apologize for my lack of absolute test results there.
Am I looking at a switch issue or something more sinister? If it matters, the switch is the same as this one (minus the Rear Fan control). It's not my truck so I'm mostly just curious about this.
EDIT for
@GLTHFJ60: I knew you'd say something like that - you just have that air about you. Below pic stolen from the internets.