No air blowing from upper vents

1997 Savana Van, 5.7L V-8, many, many, many miles...230K +.
Has been a very dependable car, with the exception of fuel pumps which leave you walking....but that is not my current problem.
Recently, the A/C started cutting out while climbing steep grades, and blowing cold again while downhill or level. Then all of a sudden, no air out of the upper vents. Does not blow out on A/C or vent..but some air down low. Also, can feel the compressor come on, can feel a small amount of air at lower vents...but none up high. Rear is working fine. The fan switch works, at least you can hear the fan going through all the speeds, but none comes out at the upper vents. Can onyone help with this issue?

I think this vehicle has a vacuum supply for the heater case. I would start at the engine with a vacuum gage and follow the air line to the dial control for the heater. My 2000 silverado has a vacuum switch in the heater control.
Make sure this is getting vacuum.

You have described the troublesome mode door vacuum leak to a "T" Your air flow mode doors will divert all air to defrost when there is a catastrophic loss of vacuum supply- this is for safety reasons- to clear the windshield, to be able to see to drive. The air flow going to the floor is halfway there. The reason the air changes modes when driving up a long hill etc, is because there is less total vacuum available from the engine- ( your throttle is opened further) and the leak is now more prevalent then ever (less total vacuum in-hg available)

Check your black plastic vacuum line underhood at the engine intake manifold- and follow it to the vacuum reservoir- look for broken, cracked, burnt places- where it is sucking air. 95% of the ones I see are at the intake manifold where heat has caused the problem- brittle lines, or cracked- also anytime a mechanic has been in there- there is a chance he moved and inadvertantly broke a brittle line. Good Luck

Not 100% sure if these are the same, but some vans had the efrost vent middle of dash, oval in shape. Pop it out and there is a vacuum pod and line underneath. I have often had to fix this line as it pops off. Good luck, CHEERS