Don't pull the cat, you just need to do a pressure test before and after to see if there's a difference, small hole for the test, and a quick buzz up to patch it. I doubt it's the cause though, I've never seen one plugged on a Toyota even the one I have with over 500k miles isn't plugged. Cats normally don't go bad, unless there's another cause like bad O2 (dumping way too much gas) and it takes a while at that, pretty uncommon to find.

My money is on the already mentioned head gasket, do a compression test on each cylinder, I bet 6 will be low and maybe other's bad too. Another check is to pull the spark plugs, they all should be the same and kind of a light brown type of color. If any are super clean (while miles on them), there probably is a head gasket leak where coolant is getting in the cylinder. Over time it will wash out your cylinder and require a bore increase to properly fix the engine. Either case fix the head gasket asap since it will fail sooner rather than later.

For the mention of the valves, I've heard the 3.0L (what the 3.4L is based on) has an issue with burning exhaust valves. I don't think the 3.4L has that problem often, but could be possible. While you do your headgasket, I'd defo check out the head/valves too. Wouldn't hurt to have the valves reground and such.

You have a service manual, did the old coil packs test bad or something? Never blindly replace parts, you're just throwing money away for no reason. I'm personally not a big fan of "off brand" service manuals, but you already have it so hopefully it's accurate. I have Toyota EWD (Engine wire diagrams) that I could probably pull some specs from if you wanted to double check.

Air flow meter has a built in IAT sensor, testing the part should show it's out of spec, and you can test the new part to validate it doesn't have the same issue.

These trucks use really reliable parts, sometimes it's as simple as a broken wire or a poor connection. Japanese electronics rarely fail from what I've seen except in a couple of cases (the dash clock comes to mind with it's odd internal spring style connector for board to plug connection). Also on the replacement parts, I personally always run OE/OEM parts except in rare cases where I don't care much. Denso/NGK for plugs/wires, NSK for bearings etc.

 

Fix your known problems first. It sound like you are sure you have a slight head gasket leak, it NEEDS to be fixed before it gets worse and if it fixes your other issues, BONUS. If not you can eliminate a lot of possibilities while you are there.

 

 

Makes sense to me, I never ran into it personally, but my dad has had to replaced some plugged ones since he's a mechanic. I think they generally fail from the O2 sensor being bad and dumping too much fuel. Seems so many people don't care about the O2 sensors when it's a pretty critical sensor for a happy engine.

 

Woe thou doth not change a/f sensor in time...in my case.

I would think minor head gasket leak is or will be a major issue. Agree with previous recommendations.