check the frame around the rear spring hangers, if the body is rusted that bad the frame prolly isnt in too good of shape either. having a solid frame is much more important than a pretty body.
if the frame is good, the only real way to fix rust holes is to grind out every last bit of rust and weld in new sheet. sometimes it is more cost effective and quicker to find a YJ tub and swap it on instead of trying to fight the rust montser.
"character is doing the right thing even if nobody is looking"