I'm surprised nobody makes a pre-cut gusset-plate that could be welded to the outside (rear) of the frame to strenghten it BEFORE it gives way to rust.
If it started at the rear of the body-mount that's just behind the transfer case skid, it could extend back UP the curve of the frame to the spring-perch for the rear axle and encompass the area that cracks and pulls away, thereby fixing the problem for good.
Once the frame cracks and "moves" it would be pretty tough to get it back to the exact location to fix it, but if you perform this mod BEFORE any cracks happens, you'd buy your frame another 20+ years.
I intend to live forever. So far, so good.
'99 Wrangler Sport: 2.5" Old Man Emo lift and shocks, adjustable trackbars, Quick-Disko sway links, rollin on 31" BFG A/Ts