I have a '95 Dodge. A couple of years ago, the AC, heat and defrost flows to the passenger side of the vehicle shut down. Problem was with the electro/pneumatic controller (the box in behind the dash push button controls). Some corrosion and line plugging within the box required it be replaced. New ones can be tough to find (the mechanic said he might have difficulty in finding a unit for such a "vintage" truck), but managed to locate one and no problems since. I agree it does sound like a problem in the vacuum system.
Les