If the main throttle plate (and connected linkage) is moving freely and snapping consistently to the closed position - and you have checked this out many times - you could safely eliminate that system. Also you have replaced the IAC with a known good unit and checked for air leakage under the throttle plate.
Without considering bizarre scenarios, idle speed is mainly affected by air flow/volume.
You describe it as only happening while the truck is in motion. Are you sure that you can't duplicate this condition with the transmission in neutral and the truck motionless?
If you could do that, it would be much easier to identify what is allowing the excess air flow.
No codes showing, correct?
Try this to be sure it isn't an air leak under the plate:
Let the truck come down to normal idle. Shut it down. Disconnect the IAC electrically. Drive around like normal and see if the problem continues (you will have to control the idle speed manually with the main).
Then, the idle air control valve will be disabled and in the "parked" position (parked position = almost fully closed with only the minimum opening allowed). Consequently, any high idle condition can be isolated to air leakage under the throttle plate (i.e. intake, TB gaskets etc.) - but this is only true provided that the main throttle is definitely always closing fully when the accelerator is released.
If the idle no longer hangs up high - you can assume an IAC problem (either mechanically within the TB or a control issue from the ECM).
