I have already read the previous threads about sticking throttles on these trucks... But my problem seems to be a little different from the ones already listed.
My 1999 GMC has a 4.3 and a 5-speed. While driving, if I lift off the throttle it doesn't slow down. It is almost like the cruise control is on (it isn't). It also does not decellerate when I shift. It will stay at whatever rpm I was at before I pressed the clutch in. But, if I come to a stop, it goes right down to idle RPM. Any help would be appreciated!

I know you've read this on all the other posts, but you might want to try cleaning the throttle body, it might have some carbon in it.

Definately cleaning the trottle body with a spray choke cleaner and a soft tooth brush should help you out.
Also those engines will slowly return to idle speed between shifts...I believe the calibrations in the PCM hold the IAC valve open while driving to prevent stalls.....normal operation

Well, I took the throttle body off and cleaned it. It didn't make any difference. My girlfriend drove it yesterday and freaked out cause it kept going after she let off the gas. I don't know why, if there is a problem, the computer doesn't trip the check engine light or something. Is there any way to check the throttle position sensor? How about the speed sensor? I hate to just start replacing things.... Would an OBD scanner tell me anything, and what is a cheap but good one of those? What about a real-time setup? Something I could hook up to a laptop and drive down the road to see what it is doing?