"chopper" modules are either a large transistor
or a bank of FET's on a heatsink
they "chop" the motor voltage creating the PWM to control motor speed

they probably failed open (fried)

some trucks they are external others they are internal (in the card)
i would assume for steering they would be in the steer card

i dont know this machine but i would try to track the steer motor cables back to its controller

id also go get the book if it was me