There have been issues with the TPS
harness connector regarding your year and model:
Article No. 00-3-5
HIGH IDLE - TAURUS WITH 3.4L SHO AND RANGER WITH 2.5L AND MANUAL TRANSMISSION
Publication Date: JANUARY 31, 2000
FORD: 1998-99 TAURUS SHO
1998-2000 RANGER
ISSUE:
A high idle may occur on some vehicles. This may be caused by a poor electrical connection at the wire harness (-12B637-) to Throttle Position Sensor (TPS).
ACTION:
Splice into wire harness (-12B637-) just after the TPS with TPS Connector High Idle Service Kit. Refer to the following Service Procedure for details.
SERVICE PROCEDURE
Verify the concern vehicle exhibits a "high idle" condition when depressing the clutch or shifting between gears for the Ranger or when shifting between Park and/or Neutral for Taurus SHO. If high idle condition is not exhibited, proceed with other driveability diagnostic fix procedures. If condition is present, obtain a High Idle Service Kit (XF1Z-15K607-AA) and proceed by following the instructions found within the service kit.
PART NUMBER PART NAME
XF1Z-15K607-AA High Idle Service Kit
WARRANTY STATUS:
Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage And Emission Warranty Coverage
And yes, I would also suspect the IAC valve, which is responsible for maintaining proper idle during deceleration.