Ok, I must confess... I didn't listen to the vid till now, I assume you think it's coming from the diff? The double-clunk is in rhythm with the 2 driveshaft bolts, maybe a coincidence, probably not.
Don't know about the rattle. Freaky.
Ask one of the custom Driveshaft guys to give a listen.
I'd like to see the inside of the new driveshaft coupler design, [edit] found it, its a poly or rubber joint, not much there to make a metal rattling noise.
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lib·er·ty
/ˈlibərdē/
noun
1.
the state of being free within society from oppressive restrictions imposed by authority on one's way of life, behavior, or political views