Did you turn the driveshaft by hand to narrow down the location of the noise?

Did you try to push up on the rear of the trans to see if the mount is separating?

Did you look around the trans tunnel, fore and aft, to see if there are any witness marks? Do the same with the exhaust.

Inspect the rear suspension as it make be the source due to loading and unloading as you shift.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.