I would have to say vacumm leak as well. My current truck and the 94 F150 I had before it would do pretty much the same thing, but it would change from vent to defrost, then let off the gas and it would switch back. I don't know about Rangers, but the problem on mine has always been the lines on the passenger side near the firewall. They are the little white 1/8" lines, and they just get brittle and crumble in places. I just found some small hose that would fit over them fairly tight then cut out the bad section and just pushed the old hose into the new section or you could just replace entire length of existing hose, but I found some have molded on rubber connectors on the ends.

TC