After truck began to run really rough, took it to parts store where they ran test on check engine light. Results--random misfire. Probable causes--plugs, wires, coil, vacuum leak, injector fault, high or low fuel pressure. How do I narrow this down? Nothing about the O2.

it could be all of those or just one of those. since you getting a misfire does it tell you which cylinder you're getting it on, or is it random throughout all cylinders?

It just said random cylinders. I just had the heads machined and gaskets replaced two weeks ago, so I don't need another expensive repair.

Start cheap, plugs and wires first. You can put a guage on the fuel rail to check pump pressure. Vaccum leak usally only bothers at idle, unless it's a large one. Then you should be able to hear it. You can try taking a spray bottle with water in it and mist the wires with the truck running, If you see an arc from any of the wires they need to be replaced. They usally bother more when it's raining or first thing in the morning before the dew has lifted. It's usally easier to see if you do it in the dark too.

Did it do it while driving, or at idle? You mentioned you just had the heads done, so I assume its not a burned valve.

My truck had the same sypmtoms (flashing CEL while it was setting the code) and turned out to be a clogged CAT causing it. do you hear a rattling in your catalytic converter at all?

if its missing on all 8 i would say its not plugs or wire. i would look at cap and rotor first. if not those maybe check the coil.

The coils are known for going on those trucks, They're right on the front of the engine. I had a '01 I replaced the stock coil with a chrome round one and mounted it on the passenger fender well instead.

yea my stock was was starting to pull apart as it was delaminating so i bought the accel coil for it and also moved it to the fender.

I had the same issue and let me tell you these guys on here helped me out alot. This was MY fix I'm not saying it's yours but we had the same symptoms. Just like "LAST OF THE 5.9's stated start cheap= cap, rotor, plugs, and wires. Make sure you route those wires correctly because if you don't you'll still get the missfires. If this doesn't work then replace coil and check fuel pressure. After that then run some fuel injector cleaner through a few tankfuls of gas to rule out injector problems. After that then replace the pickup coil ( I went ahead and replaced both the pickup coil and crank sensor ) while replacing the cap and rotor check the distributor to make sure it doesn't wobble around this can cause random misfires as well. I use the E3 spark plugs, Accel high performance coil, and Jeg's 8.5mm spark plug wires. So far so good, no misses or CEL's and the truck runs like a top. PARTS CHASER mentioned a clogged CAT, take your rig to any reputable muffler shop and have them check the cat, they'll do it for about $10. They punch a small hole in front of each inlet pipe and just behind the outlet pipe. While someone is in the truck they stick a measuring type tool in each hole as the assnt is reving your engine up to a predetermined RPM. They then read the data and can let you know right then and there whether or not your CAT is clogged. My back pressure was 1 so it was fine. I know this is alot but I'm trying to give you some ideas that hopefully won't cause you almost 6 months of frustration like I went though. Good luck.

oh one more thing if you do decide to get a new coil. before you reinstall it go the the hardware store and get 2 new bolt that are hex. mine had phillips head ones holding it one and they sucked to get out!