I see that there is an adjustment screw because I dropped a piece of rubber inbetween the screw and cable and it doesn't idle down too far anymore. How do I adjust this without taking everything off? The head is underneath

 

That adjustment screw is NOT for the idle speed but to keep the throttle plate from digging into the throttle body. Idle speed is computer controlled by the TPS and IAC. If you don't know what those are please do your homework before opening the hood.
BTW, this is a Cherokee forum not a Wrangler forum. While there are some similar parts and systems you'll get better responses and assistance from fellow Wrangler owners on a Wrangler forum.
Best of luck.