My Jeep uses the same type of system, but of course all the parts are in different places. I had the exact same set of symptoms, and in my case there was a cracked rubber fitting between the vacuum line and the reservoir (the “coffee can” mentioned above) that allowed the line to pull 7-8 in of vacuum but not the full amount; this was enough to hold the HVAC in the right spot for normal cruising but it would change to defrost on acceleration. FWIW my Jeep’s ‘coffee can,’ which is a black plastic tank about the size of a pint or quart, is behind the passenger side fender. Doing a google image search for “ford vacuum reservoir” might help you visually ID the part.
So, in my case it was an extremely cheap fix; the challenge for me was not only tracing all the vacuum lines, but finding the leak which was literally a hairline crack in a rubber fitting.