How to repair unibody crack at rear seat latch

[xcm]

If you want your 200,000 mile Cherokee on bigger tires to stop cracking its sheet metal, stop driving it off road.
It’s unavoidable, short of fully caging it, and plating the frame. The Cherokee unibody is a beer can. I don’t get why it’s such a big deal.

 

[maxbraketorque]

I have a simular crack under my rear seat

...

I am trying to picture what motion can cause this crack. I am thinking flex of the rear end of jeep up and down might do this. Prior to adding the center stiffener, that kind of bending might have been absorbed over the full wheel base, but with the center stiffeners installed, well the center dont flex much now, cant absorb the stress by flexing, so all the stress now is getting concentrated behind the rear end of the center stiffeners, which is under the rear seat.

I got 33x 12.5 tires 5.5 inch lift, 4.11 transfercase low range, Ford 9 inch rear.

Now if I do rear frame stiffeners and tie them in well with the center stiffener, then this area should see a lot less bending from up and down motion. However if this crack is from a twisting force (as in one rear wheel up, other rear wheel down) then rear stiffeners may not be solution, rather some sort of cross member to withstand the twist maybe needed.

What say you all?

The XJ unibody twists and bows. The front doors become significantly misaligned from bowing simply by supporting the front of my XJ on the unibody rails just below the door hinges. The crack under the rear seat is likely due to twist. Seems reasonable that unibody rail stiffeners will reduce bow, but sure how much they reduce twist. They should help at least a little, and people who have installed full stiffeners say there is much less trim panel creaking. Caging will reduce bow and twist. I wonder whether good rock rails that tie the unibody rail to the spot weld seam along the bottom-side of the body will help reduce twist. It should help some, but I have no idea whether it helps a little or a lot. I suspect that it only marginally helps.