Had a similar problem on my 2.6. There are two limit switches that must be closed to signal the ECU to command idle speed. One is the microswitch in the linkage and the other is the throttle position switch at the end of the throttle valve shaft. Test them both for continuity - they should be closed when the throttle is closed.
Disconnecting the harness connector from the throttle valve switch and checking for continuity showed that it was not closing. It's held to the throttle body housing with a couple of phillips head screws, which would not be easy to remove in those confines, so I douched it thoroughly with contact cleaner and kept cycling it by opening and closing the throttle. It eventually started registering continuity with the throttle closed, and the idle control speed returned to normal.
That was about a year ago and the idle speed control function is still working properly.