Deep rumbling when truck loaded (with cargo or TT)

It's really great when the OP reports back with the solution, or apparent solution. Thanks for that.

Most never bother, making all of us wonder if something ever got resolved. If this has resolved your issue, it strengthens those who pointed you at a driveshaft / driveline issue. It shows who was good at diagnosing it, and helps for similar future situations. And, now you can speak from experience too.
Kudos to Jimlin

Do let us know how it is after a few runs under the same circumstances.
:):)