There are several different lock nuts available from most any hardware or home improvement store. Nylocs ... distorted thread ... interfering thread nuts just to name some. I prefer the nyloc nuts as they will not back off. No grease worries, no need to Loctite these, and no need to have some double nut concoction that looks like something you'd see on the Clampett Moblie. 30 - 50 ft lbs on the shackle bolts
https://www.lci1.com/assets/content/...ifications.pdf