Check to see if the wheels are damaged, it's possible one of the tires got ripped while dominating those rocks. If so, check out our Wheel Replacement Guide.
If the wheels are all fine, make sure that they are spinning at the same speed. If they are not, the motors are operating at different speeds so you can try two things. First, try replacing the batteries. Low batteries can cause the motors to respond abnormally. If that doesn't work, see our Axle Replacement Guide.