My first thought was that it sounds like something going on with the rear driver side leaf springs (just due to the nature of the noise, sounds like large thick metal) but can't get it to make the noise when I'm not driving and I can't see anything apparent wrong with them. Its definitely toward the rear, and sounds like it's on the driver side but could be diff area too.Several things to look at, broken/rusted u-bolt, broken/rusted center pin for the leaf pack, transfer case seal/sleeve can make noise, or could be internal diff noise. All plays into how much digging and reporting back here you want to do along with any other input that might become available.
The fact that I can't get it to happen without driving has me wondering if it is something with the diff, but it's also a 2500 so doesn't budge much without weight in the bed even when you jack up one side and drop it. When I get a minute I might try to find somewhere on my property I can get it to reproduce the noise and take more notes of the position.
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