Re-welding that nut or a new one is what I would prefer,(if it was my Jeep), but I'm not sure how it could be done. Welding from the bottom, inside the tube looks like it would be a PITA, if it's even possible. Maybe it can be welded from the top? It's been too long since I replaced my bumper and I don't have any pictures of what it looks like without the it. But the top of that tube is flat, so FWIW, you could find a spacer or stack enough washers (because of the gap between the frame & the tube) to be even with the inside top/flat part of the tube and then add a larger washer or cut a piece of 1/4" flat stock to fit. Then just use a longer bolt & nut.