The transmission was rebuilt not too long ago, it is time to change it out (maintenance interval). Usually would take it in to have done, but with time on my hands, will give it a go.Unless you have been changing the transmission fluid on a regular basis, this may not be a good idea with that many miles on it.
The one they have will just read the codes. The one needed has the ability to manipulate the ECM (computer) to reset the idle and throttle. Had thought about buying one, but don't have the necessary 4-5K lying around. If I did, sure would get a nice thermal instead
since you changed the intake . You probably have a vacuum leak . Spray some wd40 around all the gasket surfaces . if your idle changes then you have found the problem . That 2000 is a LS motor with a plastic type manifold . over tightening or the seal slipping will be a problem or it can even be a crack . When i worked at the GM dealer we had some manifold issues with hair line cracks.
must have taken it to the dealer . They have the software to update the ECM . I still have the TECH 2 and the software i used when i worked at allen samuels in houston.