If defrost is not working it ISNT a vacuum issue of the normal kind. Whenever you start your jeep it's on defrost as that's the default. If you have a major (ish) vacuum leak it will stay on defrost forever. But if defrost isn't working and you feel air blowing out the cowl, you need to apologize to your lady friend for blaming that pack of m&ms that got really light that was on your dash. Jks but your cowl "could" be plugged, but I'm leaning towards something going on in the dash. Something probably has your blend door all flunked up, not letting it back to its neutral position.