yeah that 'electric box' you speak of is actually the throttle body control... it has gears in it and a servo that the TPS sends signals to to tell it where to position itself according to where the pedal is.
What i've seen in the past was the inside where the servo and gears are tend to dry out, the grease drys up and it starts sticking or hangs up a little bit. Or the wires break at the plug or just inside the box.

If you had the MCFA diagnostic software you could plug into it and monitor the voltages at the TPS and make sure it's set correctly. If you have adjusted on it any that may be why the idle is so high. Just remember though, if it gets too far out of range it will not rev up at all and put the engine in limp mode till it gets set where it's supposed to be.

plus that serial number is incorrect
i believe it should be AT82D02751 :o)

the fuel system on that truck should be the SG15 Carb1 Tier1 system. I do have some papers on how to check that throttle body and accelerator and a few other things.
email addy?