There is a company that makes specific 4Runner frame pieces for weld-in repairs, don't recall name but someone will chime in. There's youtube videos too.
I recently watched a Youtube video: "Toyota Pickup frame replacement w/o removing engine/trans". This could be done on a 4Runner too, I'm sure, and would make it doable for a lot of us.
don't sink money into it and figure out the quickest way to maximize the remaining value of the truck. The effort won't be worth it. The most I'd ever consider is to find one with a blown motor/trans and swap it over to the rust-free broken one.