They did tell me which cylinders but honestly I forget. I was to say 2 and 4 and another had super low compression. The one thing that was odd to me that I forgot to ask about was the fact that when it started acting up on the highway I pulled over and it was idling super rough. When it was sputtering enough like it was on the verge of stalling if I put it into neutral it ran fine. Which I would have thought pointed to an electrical issue. However, when I started it up to back it out and get it on the flatbed it ran rough regardless of whether it was in park or not. Maybe putting it into neutral when it first started was just a fluke that it ran ok......They also mentioned that they could dig into the engine and verify if it's lifters or cylinder heads or pistons(forgive me, not a mechanic when it comes to internals), but at that point the cost could just keep going up and up, and it may end up being cheaper to just get a replacement engine. Of course the first thought that entered my mind was, if I'm doing that I may as well put the 6.2 in it and swap out the transmission too. lol. Probably not that easy though. I also found another post that stated:
"About 30K miles ago, with about 95K on the clock, with our 5.3L 2007 Yukon, I noticed a low idle miss. The dealer debug started with the PCV and along the way found it didn't have the valve cover upgrade. They did some more diagnostics and determined I was getting some oiling on a plug primarily due to a piston ring issue.
GM paid for all the parts (new valve cover, 8 pistons, 8 ring sets, etc., etc. etc.) My dealer installed it all very cost effectively for me. Great support from GM. Zero issues since, runs like a top."
This was from December of last year so I confirmed that this was actually done at that point with their truck that far out of warranty and they said yes.....