Since the trailer hitch is connected to the frame the vibration will travel along the frame. The closer the problem is to the frame the more likely you would be able to feel it at the trailer hitch.

I would check the upper then the lower suspension components for bad bushings, ball joints, shocks, sway bar links etc. That would include the sub frame bushings even though there are indications that is not the problem.

I would get it up on a lift and see if it will clunk that way. Probably won't because you won't have load when you back off the gas. Braking might make it happen. Mechanic didn't have any idea where the clunk was originating? Could be the rear diff also. So many options, need to keep troubleshooting to narrow it done. Not unusual that when you can hear a clunk you can visually find it without making it happen. Spare is locked down I assume.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas