I Can't really see what you might find from switching off the engine while driving. If you have checked the car over properly while it was on your drive way, and if every thing like the Charger, air filter, exhaust, ect are ok, then it sounds like a part that works when it is in motion, that is the problem.
I doubt it would be a suspension, CV, or a wheel bearing problem, as they make knocking sounds when they fail and this usually happens when you are steering.
It could be the gear box ? Is your car manual or automatic ?
Try driving at your crusing speed again, but this time take it out of gear, or put it into neutral so you are coasting down the road. If the sound stops then it is your gear box, if it continues, then I'm guessing it's your diff.