Well I guess it's gonna cost me $100 to have them welded on. No one in local FB buy/sell groups is interested in the job it seems. I contacted a shop in Amarillo that is $150 minimum which seems rather expensive.
Today though, I happened to drive past a metal shop here in this town and their FB page has pics of custom gates and such things, so it was obvious they do welding. They responded on FB and said about $100 to do it based on the photo below that I showed them. So, I guess that's what I'll do. I was expecting it to be about $50-80 but if that's what people around here charge, so be it. I don't want to put it off anymore and it needs a professional to do it, being such a heavy duty application.
It's gonna make the rear contact the ground more often since the caster will reduce the ground clearance a couple inches vs the skid, but at least with the casters, it'll roll instead of drag.
Minor annoyance that I need to wait until Monday now though, since they close at 3:30 and I only thought to contact them a bit ago at 3 on a Friday (today). They said drop by and they'll try to fit me in, so.... ok.