Since expeditions have variable assist steering, the sensor is mounted to the steering column under the dash instead of being in the engine compartment. Look under the dash for a wheel with slots in it all around. That's the sensor wheel that passes through the sensor as you spin the steering wheel.

I'd be more inclined to think the IAC valve is just being a bit sticky, rather than think the sensor is all of the sudden no good. Of course it's possible that some wiring was damaged during the engine swap.