In my 2002, when driving up a hill, the a/c will change from the vents to the defrost. Once I get up the hill, it will switch back to the vents. Sounds like some kind of vacuum issue. It only does it if I stay off the gas and make the truck stay in o/d. If I use the button to turn o/d off or give it enough gas to down shift, then it will stay on the vents. I've also noticed a hissing sound coming from under the dash when I initially press the gas. Could these two be related? Maybe a leaking vacuum reservoir, or something?