So, unfamiliar with the DBXL, I stripped the front end down, no issues found, just a massive diff in tip top condition. Didn't open the diff itself, it's turning fine. Reassembled.
Then noticed movement at the motor, it's coming loose. Ah, could that cause the crunching? Applied threadlock, Reassembled, still crunching.
OK, rear diff is coming out. Much easier than the front! There's the problem, the diff housing is coming loose and allowing the pinion to move. Threadlocked and Reassembled, crunching has stopped!
I'll go looking for a new diff and perhaps a new housing, ally?
Forgotten how much I enjoy the tinkering, daunting at first when you're unfamiliar with the truck, but really enjoyed finding and fixing the issue.