AC problem

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The air management under the dash is all controlled by pneumatic valves operated by the knob. It's a system that hardly ever breaks, but when it does, it's hard to fix without just replacing stuff.

You're sure the knob is set to just A/C when it's blowing out the defroster ducts? The A/C is supposed to run when you have the defroster on, but air just comes out the defroster ducts. When the knob is set to normal A/C, air only comes out of the front dash vents. If that's not happening, something is screwed up with either the knob assembly or those pneumatic flappers that open and close and control where the air goes.
I'd say one of the hoses popped off the pneumatic cylinder that moves the air diverter thingey. You might get up under the dash and look for any loose hoses. If I remember right, they're all color coded. I've never heard of one of those cylinders going bad- they last forever!
This function is controlled by vacuum from the engine. Several things can cause these symptoms: vacuum hose failed/off/broken, switch failure, or a stuck valve under the dash. Btw, I have these identical indications on my 97 V6 but I haven't fully diagnosed the problem. I really doubt that it is the switch. I had mine out and it is nothing more than a sliding switch that covers/uncovers ports to alloow the vacuum to "do its thing" and direct the flow to the appropriate area. Btw, in my case, all I get is air out of the defrost vents but it is not air conditioned air. You can verify yours to see if you are getting plain old air or a/c by switching it on and have someone look at the a/c clutch. If the clutch doesn't engage, you aren't getting a/c.

Anyone have ideas on which of the myriad of lines under the hood feeds the vacuum to the switch?