Q: Car begins to shake at or about 50 mph. It's almost like a motor mount maybe broken. Could that be the issue?

Hello there, many faults will cause a vibration above 50MPH in your 2009 Chevrolet Malibu. It is great you have noticed the vibration is not in the steering wheel, as this rules out potential issues. The vibration is not likely due to your engine mounts, engine mounts can be felt as a jerk or clunk when the vehicle is accelerated or put into gear. Common faults that will cause a vibration are tire balance, failing tire, wheel bearings, ball joints, or suspension bushings. A qualified Technician such as one from YourMechanic will be able to diagnose your vibration issues and make any repairs required.