After engine is fully warmed up and idling, unplug the IAC Valve's 2 wire connector
Idle should drop to 500rpms or so, or engine may even stall, either is GOOD, it means no vacuum leaks
If idle stays high then there is a leak, assuming you haven't adjusted the Anti-diesel screw on the throttle linkage
With IAC Valve still unplugged you can pull vacuum hoses on the intake and cover the ports with your finger, if RPMs drop you found the system with the leak
Or spray soapy water around the intake, it should stall out the idling engine if its sucked in, and safer than spraying flammable liquids on a hot engine
If idle does drop with IAC Valve unplugged then the Computer is setting the high idle.
Could be ECT sensor is reporting cold engine all the time, although that should set a code.
Could be a glitch in the software, it occurs over time, you can give a Ford Dealer your VIN and tell them its a high idle issue, they can tell you if there is a software update available, this was not all that uncommon of Fords, or GMs for that matter.
You do have to pay for the time it takes to do the update but its the only fix if it's a software issue.