+1 What Edgethis said.

A dirty TB can have crud in it causing the butterflies to be held open in the bores.

One check you can do prior to running KOEO and KOER since you have already determined there are no vacuum leaks (which is critical for the following test), is to warm the truck to normal op. temp. (about 10-15 minutes at idle or about 2 minutes at 2000+ rpm) and check that the IAC is functioning properly.

This test also determines whether or not the throttle plate stop screw has ever been messed with or there is crud in the throttle bores keeping the butterflies from fully closing.

With the engine warm and idling, disconnect the IAC solenoid from its electrical connection to the engine wiring harness (two-wire connector near the IAC). As soon as the connection is broken the engine should begin to wind down and it should die within a couple of seconds. If it does, turn the key off and reconnect the IAC. (the problem is not related to the IAC and air intake). If it does not wind down and die within a few seconds but continues to idle, you either have a vacuum leak (which you know you don't) or for some reason the throttle plates are not fully seating in the throttle bores. This can be caused by coking of the interior throttle bore walls, the throttle butterfly plates themselves or the throttle plate stop screw has been mis-aligned at some point perviously. The first project should be to thoroughly clean the throttle body with a non-abrasive cleaner that is safe for both catalytic converters and "sealed throttle bodies". If the problem still exists after cleaning the TB, the throttle plate stop screw should be closely examined to determine if it is indeed holding the throttle plates open. (This mis-alignment is often done by owners and/or technicians who believe they have found a way to resolve low idle or rough idle conditions by forcing the stop screw to open the throttle. Noting could be further from correct. The stop screw is there for the sole purpose of keeping the throttle butterflies from sticking in the bores when they snap shut. Forcing the stop screw to hold the throttle open completely defeats the EEC-IV's ability to control the idle).

Just so you are aware... having disconnected the IAC will trigger a fault code that turns on the Check Engine Light so when you get the code reader be aware that there is a potential for Codes 48, 81, 99 to be present if your ECM produces two-digit Codes. There is a potential for Codes 121, 123 and 543 if your ECM produces three-digit Codes. If any of these Codes are present it is likely due to the disconnection of the IAC for this test and should clear without further incident once you have completed the KOEO and KOER tests and cleared memory.

Any OTHER Codes should be troubleshot as necessary.