BlueTeeJay, your problem is slightly different from what I was experiencing, but the write up below should help you troubleshoot your problem. I have posted this write up several times. It will walk you through a lot of the TJ vacuum system for the climate controls.
Crawl under the dash on your drivers side and watch the vacuum activated switches as the dash switch is changed. If necessary post your observations. It sounds like the flapper is not moving. This could be due to either the flapper itself being jammed, or a vacuum leak either in or before the switches.
I hope this helps anyone whose vents and or heat doesn't blow, or anyone whose vents stop working when entering the highway or going up a steep hill.
I am the original owner of a 1997 TJ. I (also known as the loose nut on the steering wheel) am one of the few original pieces left on my Jeep. Recently I have discovered several other pieces of original equipment, all of which control the direction of air from my vents and heater. My tale starts about five years ago when my dashboard vents stopped blowing. My under dash heater and defroster still worked, so I just blew the problem off. This year my under dash heater stopped working, leaving me with only the defroster. Something had to be done.
I started at the control switch. I found the vacuum lines attached to the back of it, and blew them out with a football needle and my garage air compressor. I also found a splice in the line behind the glove box. I cleaned that out as well. I insured that everything was correctly seated and tried the vents and heater. No dice.
Undeterred, I started poking around under the dash. I discovered three vacuum activated switches with arms attached that control the air direction. They did not budge when the control was set to either vent or heat, and the input to them showed vacuum on the gauge. I was able to manually move them, thereby changing the direction of the air. I knew I was close. I contorted myself into a two hundred and fifty pound pretzel, and changed them out. This took about an hour, and cost about a hundred dollars. To my great disdain, there was still no air blowing through the vents or under dash heater. The switches at least tried to move (a definite improvement) but it seemed the vacuum was not quite strong enough to move the arms.
I crawled back under the dash and traced all the vacuum lines, identifying the black one as the source vacuum. It penetrated the firewall on the passenger side, not far from the battery. It went to a three way junction. One of the lines from the junction went predictably to the intake manifold near the rear of the drivers side. The other line went under the battery where it was broken. I sealed it up with a nail. Upon checking, I had full control of my air direction.
This did not feel like a complete repair with a nail plugging up a mystery vacuum line, so I perused a used car lot nearby and found another 97 Wrangler. I crawled under and traced the line to a vacuum reserve container near the bottom of the Jeep. My next stop was the auto parts store for a few feet of vacuum line. The problem seemed to be fixed.
That night I went to pick up a friend to take to the bar, and when entering the highway my vents stopped working and defaulted back to the defroster. As soon as I let off the gas a little it went back to vents. I remembered the Jeep doing this many years ago when the vents worked, and remembered that it was annoying then. Now I was emotionally involved with this repair and had no intention of letting this annoyance continue. I searched the internet, and learned that I was not the only person with this problem. I could not find anyone who had posted a solution. The internet was of some help, as it directed me to a Jeep TSB that told of a one directional vacuum valve in line with the feed, but did not give a part number. I called a friend at Jeep, who informed me that he had the same problem. He hoped I was successful at finding a solution for both of us. He diligently kept working on his part the solution until he found a part number from the Cherokee, which we figured would work. Due to the remote location of my friend at Jeep, I had to stop by a local dealership to pick up the part. It cost about seven dollars. While at the dealership, I told the parts people about my problem, and asked if they had ever heard of similar problems, and they told me that the Grand Cherokee has a small reserve reservoir in line with the vacuum source supplementing the larger reserve container. For another eleven dollars I figured I would try that as well.
I got home, replaced the valve, which was a slightly different part, and installed the in line reservoir, which plugged directly into the valve. I test drove the Jeep several times on the highway, and was unable to make it fail. I was also able to turn off the motor with the key in the accessory position, and keep the vents blowing, something I donít ever remember the Jeep doing before. It is supposed to snow tomorrow morning, and I look forward to having the heat blow at me for the first time in years.