You likely have broken blend doors, and possibly a broken recirc door. These are both very common failures among 99-04 Jeep Grand Cherokees. Typically to fix the problem, a mechanic will remove the entire dash and replace the plenum assembly. There is a DIY kit that can be found online called heatertreater They provide hardware and instructions to fix the doors yourself by removing the glovebox. I put one in my jeep 3 years ago and it's still going strong.
I have a 03jgc a/c blows out the defrost & heater vent but not the dash vents. Have check vac lines seem to be ok. Possible blend door broken? Had a similar prob on my 97 wrangler, that was the case then. Anyone else having this problem. Not a dual system just basic mechanical. Any help appreciated.