car bucks when backing up
If any recent work has been performed that would be a good place to start. If any recent brake work has been performed a bolt may have been left out on a rear brake caliper, etc. It may be prevalent to place the vehicle on four jack stands and see if the car bucks when no longer on the ground. Check for anything loose in the suspension. Try to determine if it is more from the front or rear. Can you feel anything in the steering wheel, etc. when the bucking occurs?