2013 Hyundai Azera Bought New Around 72 Mph Passenger Seat Shakes Had Dealer 3

I have a 2013 Azera bought new. Around 72 mph the passenger seat shakes. Have had it to dealer 3 times. First time tires still had 60psi told they were shipped that way to keep from flat spotting. Second trip 3 of 4 tires out of balance set wit road force balancing machine. Third trip tech said yes seat does shake at 72 and above. He pulled a new one off lot drove it and was the same. They now tell me just Characteristic of car. Really? I have had it 2 weeks less than 1000 miles. Very disappointed. There has to be a fix my wife loves this car. Please help!