there are switches on the steering that (on this truck) are underneath the controller, if my memory serves correctly. if the wheel is straight ahead both motors should turn at pretty much the same speed, make sure they both have equal turning (rolling) resistance (no brakes dragging) by turning with your hands while in the air.
These "spongy" cables are part of what I was talking about earlier, as far as cables that MUST be replaced first, as they are oil soaked.
Before further troubleshooting can be done correctly. any that have the insulation cracked (visible when cable is bent) or have been saturated with corrosion must be changed too.