I really think you have bad cats !! meowLeaning towards checking the cat converters myself.. 3rd cat was fine, but that's way downstream..
You're not kidding about the starters rattling. I removed mine for motor mounts and have never felt such a loose jangling part in my hands before!
That was one of the more obvious preventative maintenance items. Cheap and easy to boot. One knock sensor is right near the back end of it..
Curious if the OP's truck drives like mine. Seems to feel normal immediately after startup, but once it warms up in a few seconds, bog...