old school trx350 foremans ?, ( 1986-1989 ) ?.Does anyone make a 350 Foreman rear axle anymore? Looks like the part is obsolete.
I know I've seen someone selling ring and pinion sets for them recently that were a special run they had custom made, but not sure about axles.
I would take it to a machine shop and have them fix it, or weld the nut on and keep an eye out for a good used one. You'll need one sooner or later when you have to replace bearings.