When breaking at higher speed the car shakes, what is causing this and how should I address this issue?
When ( BRAKING ) if the vehicle shakes it could be warped rotors or brake pads but if you can’t do brake work then have it looked at before it becomes a serious problem.
90+% of the time it is warped rotors but in rare cases it can be a bad wheel bearing, but that should also have a growling or grinding noise. On a rear wheel drive car like this it could also be driveshaft shake from worn u-joints but that would have a clunk when changing between forward and back.
That’s a heavy car with good power. If you drive and brake hard routinely, it is quite easy to have issues with warped or hot-spotted rotors. How many miles are on this car and what is your driving style?