88 D100 issues

Yeah the grease in that Idle Air Control is probably all gummed up and hard. The guy in the above video clean out the internal gear drives and relubed them and it worked great.

Unbolt the IAC from the throttle body and it will probably idle right down. Can replace it with a new part from Advance Auto Parts too. (CarQuest)

Edit:
Or did you replace it already? That plunger on there needs to move to allow it to idle down.
It is supposed to engage to speed up the engine when the AC kicks on.

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Might need to put a second throttle return spring on it to pull that IAC plunger back in at idle.

Take some WD-40 and lube up the throttle butterfly pivots, throttle cable, throttle cable and pedal inside the cab too . . (they can stick) and all the mechanical trans kickdown linkages too as they may be hanging up.

Work them back and forth a few times until they are working freely.