I was 100% CONVINCED that the noise was due to excessive axle end play. Sounded like a tennis ball rolling around in the bed at low speeds, especially while turning. My plan was to have the dealer measure the lateral play when I got my first free oil change, but deep inside I know that this would be the first step in a shitstorm of BS from the dealer. I read all of the speculation about it being improperly torqued U-boats on the leaf springs, differential, shock mounts, bushings, calipers, etc. And the most implausible of all was the idea that the leaf springs were rubbing together and causing the clunk.
I decided to take a shot and buy the SpringThings rubber spacers. Took me 15 minutes to install and I'll be goddamned, but the clunking absolutely disappeared! While under there I could see that there is a LOT of force between those upper leaves, and when they slide (its a minute amount of motion), they "pop". It's not a smooth rub like you would imagine.
Anyway, $40 well spent...I'm in love again! PS I can no longer cause the sound by shaking the bed back and forth either. For those of you caught in the endless dealer loop, or paralyzed by multiple theories, do yourself a favor and cough up the $40...better than days at the dealer, and feels like a more robust solution than the TSB shenanigans.