I'm not a fan of changing things unless the "things" are cheap, or you are reasonably sure that the problem has been properly diagnosed, and you are reasonably sure that changing said part will fix the problem. At least your dealer was honest with you about their uncertainty. Some things are just elusive, and a shop has be thread the needle described above if they are to retain customers.
If no shops near you feel that they can pinpoint the problem, know that a vibration issue like this will usually get worse before something catastrophic happens. As long as it's only under certain conditions, and not severe, wait until it gets more noticeable, or you detect a sudden change in behavior. At that point, take it back to see if it has become "pin-pointable".
This is what my mechanic had me do. It took a couple of months for my noise to get bad enough that he could warm up the drivetrain and trace it to the Front Diff while up on a hoist. At first he thought it was the rear diff, but wasn't sure. I'm glad he didn't just start rebuilding, as the front is a much easier rebuild (less labor and parts).
