where is the ac vacuum check valve for a 1996 explorer located?

Sounds like a bad HVAC Actuator under the dash if all you have is defrost.

ac woks fine but only when speeding up the air would divert to the defrost vents...then goes back to normal after maybe 30 seconds, sometimes up to a minute...

Well then the check valve is a possibility, sounds more like a small vacuum leak somewhere since you know everything does work. Should inspect all vacuum lines on the engine. There is a separate HVAC Defrost Mode Door Actuator that works off of vacuum from the engine. Inspect the vacuum line running to it, if there is a check valve in that line, and it is bad, it could cause this problem since it only allows vacuum to flow in one direction. The reservoir for air pressure would be the Actuator itself, keeping it in the proper position as you accelerate.

I had/have the same problem on my 2010 4.0 Explorer and we found a couple of problems. First, my check valve was melted and second, the hard tubing is brittle so it was broken into pieces. We removed the check valve (for now) and patched the plastic tubing with thicker rubber tubing. The A/C rediverted from the window vents to all other locations but if I go up a hill or give too much power to the engine, it returns to the window vent. I'm trying to remember where this check valve goes now that I have a replacement.

my 2008 ford explorers air is not working. Ihave replaced the actuator and still no air. the tank where where freon goes is cold but not blowing cold

I replaced the check valve and still having the same problem