[pappyl]

have an older Jimmy, but I do not not believe they've changed this too much.
- The main vaccuum from the engine goes through the firewall and connects up to the rotary control switch. This switch acts as a valve, directing vacuum to the other lines (usually 5).
1 for defrost
2 for directing middle vents
1 for fresh air vent in (diaphragm outside of firewall)
1 for a/c recirc
- each line connects to a vacuum diaphragm which (when vacuum applied) contracts a control lever, which closes a gate in the plenum (depending which diaphragm, directs air to feet, main vents, or defrost).

So what goes wrong?
- A line is disconnected, and vacuum is lost, gate does not open
- Insulation on a plenum door comes off and closes off an opening *yes this happens
- A vacuum diaphragm is faulty
- Switch is faulty.
- A spring, hinge, or control lever is faulty on a gate, which causes it to bind/not move
- someone was playing around and switched the lines at the control switch

Defrost, to my knowledge, is not "normal" state which indicates a bad gate in the plenum. "Normal" state is when all vacuum is OFF and gates are in retracted position. The air is being diverted because a gate is stuck.

Try pulling the controller, and the vacuum line connectors. Use a small patch vacuum line with 1 male ends.
Start the engine, put fan on high, and find the source vacuum. Systematically apply vacuum to each line with the patch line and listen for a door in the plenum opening, redirection of air, etc.
When nothing happens, that means you have a problem with that particular line and will probably need to yank the dash and pull the plenum (given the vacuum line has no leaks).

Applying full vacuum to a diaphragm, then disconnecting - on/off may jog it from a bound position.

Let me know if this helps.