Moss, remember that the PCM doesn't just turn the fan on when something is hot, it turns it on in anticipation of something getting hot. When you crest the hill in a lower gear the fan is running because the PCM is anticipating extra heat due to engine braking going down hill. When you shift into OD the PCM knows the engine will be doing less work and so less heat will be generated, therefore if temps are not high now, it turns the fan off.
Many have reported that the Ford dash gauges are fancy idiot lights and don't report changes as they should. Maybe get a Scan Gauge or similar that plugs into the OBD II port and monitor actual coolant and oil temps as you drive. See if you can make a correlation between temps and fan behavior.
You may have an engine condition that is creating excess heat, and the PCM orders the fan to respond to it, but the increased temps are still within range so the PCM doesn't tell the dash gauge to change.
On my truck the fan clutch is mechanical with a viscous coupling that responds to heat. As temps go up, the goo in the coupling firms up and brings the fan up to engine speed. When cooler, it lets the lets the fan freewheel to reduce engine drag.
