AC/heater vent only blow out hot air

The AC went out last winter. It started when I accelerated it seemed to cut out ( like on any dodge vehicle, when you accelerate to pass or are going up hill the ac cuts out ). It never did it before so I don't think it's a power issue. Also It didn't actually cut completely out. The air just stopped coming out of the front facing vents and only came out of the defogger/defrost vents. Until the cold air would only come out of the defog vents all the time. Then eventually the air started coming out hot all the time last winter. During the winter I didn't mind but It's summer ( in Arizona no less ) now. The weird thing is a couple of weeks back it started blowing cold air on its own. I didn't do anything to it. I thought the air coming out of the defog vents only and the controls not doing anything but switching on or off, might be a switch so I replaced all ( the fan switch on the left and the ac/heater switch on the right) except the one in the middle that switches the temp from hot to cold because it was not available in store. Can someone help me? I don't want to pay thousands to fix it, my 98 explorer sport isn't even worth that.