Followup post —
Okay, got the heep to the dealership, which is the only one I have ever trusted. (I have been a customer for ten years now, so even the older mechanics know me by sight.)
They were laughing at my rig because I had them swap the wheels off my 2003 Sport with my dead 2003 SE (31x10.5s on stock Eco rims versus 33x12.5s on MT Classic Locks). It has a small lift and Rough Country flares, so the smaller wheels with the stock backspacing look pretty silly. But the 33" MUDs are not safe to drive on and the SE ain't going anywhere with no motor. I swapped to see if the ride issues were all tire related or something else.
They are replacing both O2 sensors. The main one is going out and I decided to have them do the PITA downstream one while they had it in the bay. Get this: all six plugs had just about zero electrode left, and the nubs were almost totally rounded. The gaps were HUGE, like comically so. They were betting they could be the originals or the second set. It has 141K+ on the odo.
I asked about a general tuneup, but everything else is fine. There are no wires, but coil packs. I thought the 2003 still used a cap, rotor and wires. Whatever. What is there is solid.
So, new O2 sensors, plugs and (soon) new 33x10.5 DuraTracs on black 15x8 steelies and the MPG ought to come back up dorm my dismal 13.9 — Yes?
I included a photo of my dead SE on its final run right after the rod broke through the block. RIP, old friend. You will have a Chevy 350 one day...