This is just a hunch, but it could be your neutral safety switch (the switch that tells your computer what gear you have selected (PRND21). The switch signals the PCM so that it knows that its in gear. When in an active gear (R D 2 1) the PCM will change the air/fuel ratio and change idle speed to account for the load being introduced to the engine.

What part of TN do you reside?

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Automotive Electronics Engineer
2000 XLT 2.5L A/T
Mods:
Explorer OHC
Explorer Autodim Mirrors w/ Autolamps
Explorer Sport White Face Gauges