I have good news! I think I fixed it. Here's how:
First, I pulled the Chiltons manual. That didn't help a whole lot. The PCV valve tested good. After about thirty minutes of head-scratching, I found (what I think is) the EGR valve (thanks, Chiltons) , but it looked a PITA to get to so I let it lie for now. I started pulling spark plugs. This V-6 is a distributor-less ignition (coil-on-plug) with a twist: it has only three coil packs, with spark plug wires running to the other three spark plugs and utilizes a wasted-spark set-up. OK, fine. I had to pull a few things to get them out of the way, but getting to the plugs is WAAAY EASIER than on my old SBC. Wow, am I glad I sold the Suburban! LOL
Plugs had very little torque on them; three were snugged tight (barely), two were marginally finger tight and one was not even snugged. You know those little metal doodads that screw onto the threaded portion on the top of the spark plug? Three of those were loose, one had a little corrosion and one had a lot of corossion. Note, all the worst examples were on the driver's side cylinder bank. That's the side with spark plug wires and traditional spark plug boots, which snapped on with some positive feel but then could wiggle up and down some. Andthe worst corroded doodad was on the loosest spark plug. Hmmm.
None of this looked conclusive to me, but then, I dont' trust ignition systems for nuthin'. I replaced the two corroded doodads with new ones, tightened all six doodads, then goobered them with dielectric grease. Torqued the plugs about 1/8 turn past snug (Chilton's said 11-15 lb-ft if no crush washer, which these plugs had) and reassembled. Also, Chilton's was showing a dual-electrode plug for the V-6 and mine are single electrode Bosch platinum. Hmmm.
I had minimal hopes that it was fixed. Air filter was a little dirty so I figured I'd replace it just to eliminate it as a possibility. I was thinking I'm going to have to dig back to the EGR valve. So I hop in the truck to drive to NAPA to get the new air filter and ask to look at an EGR valve so I hopefully take apart the right thing and I can feel it just off idle, it feels better: more responsive and better torque. The misfire had been worst when lugging the engine so I lug it in second, third and fourth gears and it feels fine! It was just around the block so I'll take a little longer drive later today but I think it's fixed.
So, what was the real culprit? Ignition, obviously, but what specifically? You tell me. I think the plugs weren't torqued enough but I think the real problem was the corrosion on the spark plug doodads. What caused the corrosion? Bad connection, I guess, but one would think that the spark plug wire to spark plug doodad connection would be solid, since it's engineered to be a push-on connection. But like I said, I didn't trust it. Maybe just cleaning up the doodad and adding dielectric grease would have been enough but I just happened to have a few spare plugs for the shifter kart so I liberated a couple doodads for the truck.
Thanks to all who offered suggestions. I was thinking PCV or EGR but all you who focused on the ignition were spot-on. Way to go!
I love GRM!
David, Happy in Phoenix
PS - too all of you in the rest of the country, yes, I was forced to work on my truck in August in Phoenix! Be glad you're somewhere else right now where it isn't 101 degrees with a "real feel" of 110.