plugs are original at 119K.
Start here, as it's time for those plugs to be replaced. Get the AC Delco - both the I6 and V8 run best with them, as documented by many on this site. I prefer the iridium; others have used the dbl platinums, etc.
Actually, I see you have a Denali. Get the iridiums. I think the part # is 41-103 (but validate). Mine runs like a top with them - and I only use 87 octane, as long as it's no worse than E10 (mandated in my area). The ones I pulled out were Bosch - also iridium, and practically new - but the engine runs so much smoother with the Delcos installed. More often than not, there's usually a rebate on them, too
If you have the spend, put some new wires on it, too, since those are original.
Finally, do you tend to favor one brand / location of gas? If it's a 'Top Tier' brand, then you should be good.
The 5.3L has knock sensors in the valley cover, underneath the intake - at least my LM4 does. They're known for going south in that location (one reason why the Gen IV LS motors relocated them to the side of the block). That might be worth checking out.