While you certainly can change rear steer to the joystick, it is not a very good idea. It won't work 100% of the time and it will be super frustrating.
Rear steering works entirely different than the front steering. With front steering, you can turn the tires however far left or right you want and the joystick is perfect for that degree of sensitivity.
But with rear steering, the tires automatically turn fully to the left or the right. Having the input on the joystick makes it to where you have to perfectly move it in a certain position or it otherwise will not work at all.
Therefore, having the rear steer on the X and B buttons is your best bet. Just like real monster trucks, they use a button (a toggle switch) because they do not turn in a full 180 degrees of motion; its either the tires are turned, or they are straight. There is no "in-between" like with the front steering.
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