I just bought a '97 B2300 basically site unseen (not my brightest decision in life) but had a mechanic do some unrelated repairs, test drove it after and it's doing just as you described. If I'm anywhere around the 3k RPM range while in gear (say 3rd gear) and I push in the clutch...it just stays there until I come to a complete stop. If I'm at a lower speed in 2nd gear at around 1800-2000 RPM's and push in the clutch the RPM's shoot up and hover around 3k...it's so bizarre. We replaced the Idle Control Motor, but still doing it. What else could it be?
Throttle position sensor? Simple vacuum leak somewhere? Sticky throttle body?

Stats: High mileage (229k) rebuilt motor 35k ago

Just had clutch, master, slave cylinders replaced and ALL fluids are fresh. And now as mentioned the idle control motor is new.

 

With a vehicle this old it is a good idea to clean the TB, idle air control valve, check all vacuum lines.
I bet it is the sticky idle air control VALVE.

 

Try re-setting the computer by disconnecting the negative battery terminal followed by a few brake applications, wait 10 minutes, re-start letting it idle for 5-10 minutes, driving slowly without suddenly flooring it for 50 miles.
If that doesn't work then who knows, i would do as Tan says----disconnect battery while doing it!

Watch the video in this article: Rough Idling Issues |*MotorWeek

 

Found the problem. The throttle return spring isn't 'springing' back properly. Just added a simple $6.99 aftermarket spring to a tie wrap around the air intake plastic molding, drilled the tiniest hole on an arm that is C-clipped to the throttle body and now the RPM's come down like it should.
I'm now chasing a stupid EVAP code P1443. I've got to get this thing through emissions. I've replaced the Purge Valve, going to replace the smaller air flow thing (black) that is inline with that, as well as get a 'newer' used charcoal canister as the one that I have has tons of cracks on the top. It looks pretty weathered. If that doesn't work...smoke test.

 

Good job on the spring.
consider replacing the O ring on the gas cap. Also: the filler hose on these trucks is prone to failure.

 

Springs? They actually still have them?