It's about to give out on the other side, too. Not good!
I have seen pics of some cracked frames in that general area before. Seems like they often put a bend in the steel right about there, and it must weaken the steel. Maybe more of a stress point there from the weight
and pulling too, not sure. Anyway, some folks have caught it earlier and added reinforcements to both sides in that vicinity.
You might want to consider having a new frame fabricated, and transfer the axle
(if still good) and body to it. Some folks have done this, too. I guess it depends on how the rest of the frame looks, and whether the body has to come off anyhow to repair the old frame. Fortunately, there are some good welders out there.
I had a small cross member crack apart under my Hauley and my trailer shop was able to weld it back into place without removing the body, by cooling the underside with water. It messed up the gel coat and they painted over that, but otherwise it's fine. Of couse, my FG is a lot thicker material than yours, so YMMV.