RPM drop when placed in park on a 2001 Ford Explorer Sport Trac. Any solutions?

Check for codes and then check the operation of the "ISC" Idle Speed Controller or "IAC" Idle Air Controller. Does the Idle seem to rev high first thing in the morning when it is cool or cold out, and not go down until the auto is warm or hot ? If you answered yes then just go ahead and replace the ISC "Idle Speed Controller" or IAC "Idle Air Controller" as it is not functioning correctly (IE, it is either going bad or is bad already). The IAC or ISC is nothing more then a choke for the EFI system when the auto first starts up or starts up cold and is located on the throttle body housing.