Update to the saga. Mazda is having their master technician drive/fly down from Seattle to investigate the cause of the vibration. 100% the 2021 loaner CX-9 Signature I am driving vibrates the exact same way around 65-75 mph. IMO it's a design/assembly line or driveline flaw. I bet there's tons more cars with same problem and either the owner doesn't notice or care or was lucky enough to not get a lemon. I'll report back what the 'master' technician finds.