It may be a vacuum leak but likely could be the Blend Door Acuator, an electronic piece of the puzzle. It's a cheap part, but unfortunately a bunch of labor to fix -you have to take the dash apart, lowering it entirely. I had this happen on a '97 and my independent repair shop charged about $400 to fix it. If you are really good mechanically you could tackle it yourself, but after I looked at the tutorials, I said "no thanks!".
Theres a bunch of information on lincolnsonline.com and crownvic.net