-there are two safety switches. Look under the dash on the top of the clutch pedal you well see two. Unplug them. One will not let your car start. Leave the other one off and see if that mite solve your problem
-When you let your foot off the gas the throttle body closes. There should be no extra air going to the engine to allow it to have a high idle. The computer will open the IAC to allow more air to correct idle if it is not set right. So either your IAC is sticking or you idle needs to be re- based. Start your truck, warm it up, disconnect your IAC and then take it for a drive. If it runs fine then your IAC might be crap. If the truck does not stay running with the IAC disconnected then your idle may be set wrong.
Many different factors could come into play on why your idle may be off. It could be the heating and cooling, or the crap that is all over the butterfly in the T/B. Or you IAC could be bad. This is just where I would start. It could also be a vacuum leak, I would change all the lines that come off the tree.
Hope this helps