[dcarlson]

Like I said, the high speed is supposed to kick on when the AC is on, or if the temperature gets above a certain point.

You said it only overheats when the AC is on. That sounds like when the high speed fan is supposed to come on it doesn't. Therefore you have no fan at all, since the low speed shuts down to let the high speed take over.

Try it. Let the car warm up with the AC off. Watch the fan. As soon as it comes on, turn the AC on MAX. If the fan shuts off, there is the cause of your overheating problem. From there you need to talk to a ford electronics expert or bring it to a technician to fix.