Terriblehookah

So, this happened about 10 months ago in my 2000 F150, single cab, 2WD. My alternator went out, mid December. Me and my dad installed a new one, took us about 2-3 hours (neither of us are too auto savvy). Well after that the electrical problems were fine, however, the air from the panel, whether it may be on hot or cold, set to panel, ac, floor or whatever started coming out of the defrost vents. It only happens when I accelerate, or go up a hill, or if I have the aux on but don't have the engine running. I've looked up this extensively and I know that the ford default for the vents is defrost for safety reasons. I kept hearing about vacuum leaks online. Listened for them in the engine compartment, didn't hear anything. Did the carburetor cleaner trick, no change in idle. My dad doesn't think it's worth checking out, my girlfriends dad thinks it's then blend door. Also, I started turning off my automatic overdrive when I'm driving under 45 (as dad suggested) and it happens far less with it off (still happens, just not as often). When I bought it I lost 3rd and 4th gear so I had to get the tranny rebuilt. Based off all this I have a theory that it might be a vacuum line off the transmission, or just a problem with it getting enough power, so the engine compensates by using more air which in turn mimics a vacuum leak. I'm attending college we don't really have the money for a mechanic. Any tips, suggestions, comments? EDIT: My EGR Valve died on it, I replaced it the next day and the problem was fixed until my mother and sister took it down some back roads and drove it more on the wild side before getting it stuck on a razorback hill. I checked all the hoses and there's nothing loose or anything that I can see.

Short version: Air comes out of defrost vents at times, not sure what the problem is, don't think it's a vacuum leak, not enough money to go to a mechanic.

Also, I apologize for anything I said that sounds blatantly stupid, still learning this whole vehicle thing.