1999 Ford Taurus Cooling fans only operate in park position

I've been doing extensive work with my 1999 Ford Taurus 3.0L Vulcan OHV engine. Extensive work is as follows: Replaced front bank cylinder head (cylinder's 4-6), both head gaskets, all engine gaskets from the head gaskets up, timing chain, front oil seal, timing cover (front cover) gasket, thermostat, radiator cap, coolant temperature sensor, lower radiator hose, coolant temperature pigtail/wiring harness, serpentine/drive belt. Services done: Multiple flush and fills (radiator works correctly), heater core by-pass, chemical flush of cooling system. When the car gets hot during driving, the cooling fans never come on. All the relays are testing okay and this model year does not have the Constant Control Relay Module. If I place the car in park, the fans turn on within just a couple of seconds-this has happened four times. Problem is not that the coolant is not reaching temp for the cooling fans to come on because when the engine is killed, the coolant rises rapidly into the expansion (recovery) tank and overflows like a glass of water left too long under a running faucet. You can see the water vapor escaping from under the radiator cap. When the CTS is unplugged, the fans run continuously (I've been told this is normal because it throws a code to the Check Engine Light while unplugged), and I don't have a problem with the overflow. This is starting to kill me because I don't know where to look next. The fans come on, but only when the car is placed in the parking gear.