I just wanted to give a thanks to JeepCrawler for posting a response in a vacuum leak thread. My jeep would only blow in defrost. I replaced the AC/Heater control switch and it didn't make a difference. So I figured I had an exhaust leak. But for the life of me, after reading the online service manual (my chrysler one hasn't gotten here yet) I could not find the damn vacuum hose. After searching and reading many threads in the TJ forum, I came across this post.
Most of the time when the TJ HVAC plenum box stops routing air it's because of a vacuum leak. Open the hood and look on the intake manifold - you will see a small hose coming off of it that goes across the firewall to the passenger side. There it Tees into two routes - one going to a vacuum reservoir located below the battery, and one going through the firewall and to the selector knob you see. Inspect the hose - and experiment with blocking off the vacuum reservoir to see if the problem is before the knob, or the knob itself. You may also want to stick a vacuum gauge on there and see where it's registering. It should be around -15psi or so (normal engine vacuum), and when you rev or stop the engine (eliminating vacuum) it should stay there as there is a check valve that should keep the vacuum in (or the pressure out depending on how you look at it) and the vacuum reservoir should last a good while unless there is a leak.
I finally found the vacuum and it was just hanging on the body stablizer bar. Apparently, when I changed my exhaust header, I stuck a random wire that's not doing anything into the vacuum socket on my intake
, pull it, stuck in the correct vacuum and now everything is working great. Plus it saved me from tearing my dash apart to trace a leak. Thanks JeepCrawler98