Looking at parts look up it looks like they use a stanard washer and nut. Part numbers are 712-3018 Nut, Hex, 3/4-16 GR8 and 736-3014 WASHER-FL I would go to my local bolt supply company and replace the nut with a locking type nut. Take your old nut with you and let them know what you need it for and they will hook you up. Should be $3-4 each and I would do both sides.