What size is the part that goes into the hitch? 1 1/4 or 2 inches?
I would try to re-tap the nut, and use a bolt that you can put a lock washer and a nut on the outside of the hitch. This will do two thing, the internal nut will help keep the rack from moving around, swaying while driving. It will if it's not secure. The nut on the outside will keep the rack on your car if the threads on the internal nut fail. The down side is that it will probably take two wrenches to get the hitch off, a good theft deterrent.