Are you absolutely sure that both fans are running when the A/C is on?
On my wifes jeep liberty, it was recently overheating in the exact same manner yours is.
Every time I looked under the hood, I could see the fan working.

Well, turns out, this fan is a 2 stage fan. It comes on when the A/C is on, and should come on at a certain temperature. It wasn't coming on when at the certain temperature. So when the A/C was on, it would stay on. But every few minutes when the A/C compressor would kick off (all a/c's do this), the fan would kick off. This is supposed to give the engine time to cool off, but instead when the A/C would kick off, the car would get hotter.

Kind of hard to explain, but a new fan ($300 later), and the issue is gone.

Other than that, the thermostat is usually an easy and cheap fix. You can pull yours out and place it in to a pot of boiling water to test if it is opening correctly.