That is a nice looking truck! The original plugs in these trucks are rated for 100k (due to a divorce) I didnt change out my original plugs till 124k (they started to misfire) I had those changed out by a mechanic due to the horror stories but none broke. At the time the only single piece plug I could find was the Champion (ok but dont reccomend) I did the 3rd plug change myself with the Motorcraft (recommended) and the job was straight forward. Still loving the truck with 260k on the clock.
One thought can you call up who you purchased vehicle from and see if they changes out plugs? In most cases the COPs are good at time of change and some will opt not replace COPs (I did each time) but the boots have a tendency to crack as water and debris get into the plug wells.
Thanks JB! I still love the truck regardless of this issue. Other than my wife's RAV4, this is the first real 4WD (by real I mean that uses a transfer case with a low range) I've owned other than jeeps. I'm a huge jeep guy, and still have my Wrangler, but decided I just wanted a half ton SUV for better pulling our camper with, and eventually pulling my jeep with probably.
I'm highly doubtful the dealer replaced the plugs. They said they did a service, but it's not typically the type of dealer I'd buy a used car from. Lol. Just a whole on the wall type of place. But I loved the truck and during the test drive it ran perfectly. Even wen I floored it. No running rough, no CEL, nothing. But the dealer gave me a 3 month 4500 mile warranty with the truck at least, but with a 250 deductible. So I'm going to take it to a parts store and get it scanned so I can at least hopefully narrow down the cylinder where the misfire is coming from then pull the coil and inspect it. Probably will inspect all of them but if I at least have an idea as to the bad cylinder I'll know where to concentrate my work.
Because I had a Cadillac STS do this EXACT same thing down to each detail, I'm hopeful I'll find it's the same issue with the STS which was only a bad coil. Simple fix. But we'll see.