I have a 1999 Trailblazer, 4.3L Vortec, 142k miles. It seems as though the starter isn't disengaging, because just after starting, it makes that horrible "it's already running" sound. I did a little troubleshooting this afternoon and just bumped the starter about a dozen times, and about half the time the starter continued turning after I let off the key, and twice the starter turned when I turned the key to "on". Of course it only does this in P or N, so I'm assuming it is still ok to drive as long as I start and go with minimal idling.

Any suggestions on the source of the problem? is it the switch, starter, or possibly something else in between?

I believe the starter needs replacing.
It's very rare but what happens is the starter contacts try to weld together and not disengage when the key is released.
This situation can deteriorate to the point that the starter will stay engaged, ruin the starter wheel, heat up the starter wiring and create a general electrical wreck under the hood.

Thank you for your reply.

This morning the problem was constant, and quite a PITA to do a cold start. After driving to work the problem was still present. However, this afternoon the starter is working fine. I stopped a couple of times on the way home and even repeated bumping the starter just like yesterday with no problems.

I understand what you're saying about the contacts, but when the key is released from "start", shouldn't power be cut off from the solenoid and starter? The neutral start safety switch is obviously doing it's job by cutting power in anything other than P or N, so wouldn't the function of the ignition switch do the same? Should I rule out a faulty switch first?

After starting in park or neutral and it is still engaging does it quit trying to engaging as soon as you go to drive or reverse?
If so i would check ign switch first.
Unplug the neutral safety switch and Test the start wire from ign switch for shutting off as soon as it is turned back from start to run position.
Does it have any add on remote start or anti theft stuff?
You need to fix it as soon as you can.
Or it may eat up the flywheel.
Mt