2002 Chevrolet Trailblazer Engine running rough

Is this rough idle intermittent or does it happen every time it idles? If intermittent, can be hard to track, you have to test when problem is ongoing. The basics for rough idle, check for a leak of some kind, fuel--egr--vacuum. Check and clean the throttle body and anything that has to do with idle air system. Check for applicable trouble codes, If misfire for a particular cylinder, the code should tell you which cylinder. Some of the national brand autostores will check codes for free, otherwise, you need an obd2 code reader at least to check codes, yourself.

Just about anything can cause a p0300. I was hoping it would be cylinder specific, no luck there.

Use a gage and check proper fuel pressure. Also, see if pressure will hold for a bit with everything off, If the pressure drops like a rock look for fuel pressure leak.

Next I'd check your ignition system, how long since a tune-up?