As Zeep said, a likely spot for the vacuum leak is the take-off port on your manifold. Mine was harder to find, however. I took a hand-held vacuum pump and tested each line individually and eventually found the culprit. It was the rubber elbow on the vacuum canister in the right front wheelwell (I don't remember the exact location). It's not round like the old school canisters, but square.
Rubber hose replaced; no more HVAC problems.
2001 TJ, 4.0 Liter, 3.73s, Rough Country 2.5 Suspension Lift, JKS 1.25 BL, JKS 1" MML, Advanced Adapters SYE/Tom Woods CV Driveshaft, JKS Quicker Disconnects, JKS Track Bars, JKS Control Arms, Aussie Locker Front (Dana 30), Dana 44 Rear w/Trac-loc, Rough Country Rock Sliders, Rampage Fastback Softtop, BFG A/T 33x10.50x15, Stock Canyon Wheels.