I've been looking on a lot of blogs and been trying to see what people did to make the xj jeeps in to trucks. I know the MJ had a unibody frame under the cab and then a solid frame for the box, then it just bolts into the cabs frame. Could you just leave the frame of the xj and weld in square tubing to make the rear of the frame stronger? I know it needs to flex so when welding in the square tube, leaving gaps in the welds every so many feet or inches, would still let it move would it not? I have already cut it to give it the truck look and have the rear glass in. I just left the frame alone. The floors were gone on it so I have a clear shot right from the fire wall, all the way to the back tail gate to do the work to the frame. Any one wanna give me a idea on what to do for the frame to help it from folding in on its self?

Welcome.

Punctuation would make your post easier to read for folks here. I'm skipping over it as I'm sure many others will.

Welcome to CF.

Hope you don't mind, I edited your post to make it easier for others to understand.

Majority of folks have just chopped the top and grafted the back of an MJ cab to the XJ or they have modified and fiberglassed the rear XJ hatch to that area. That way they do not take anything away from the rear unibody/"frame" structure.

You cut the entire floor out?

Hopefully you braced the body some before that, or it will have moved.

If you Weld metal to metal, it should NOT flex. If your metal structure is flexing you have a problem. The only thing "flexing" are the bushings between the axles and control arms.

All materials deflect under load, and likewise all structures flex. It's a matter of how much. An XJ that's flexed out without frame stiffeners or a cage will easily get around 2" out of square from corner to corner, with that increasing as the unibody ages. If you look at older trucks with C channel frames you can really see this, the frame will often flex more than the suspension does (as in it may flex 10"+ from corner to corner and you will visibly see the box massively out of alignment with the cab).

There is a myth that when welding the unibody/uniframe that you must stagger the welds ("stitch weld"). Actually, we can blame Kraqa for this as his "Unibody welding bible" is still an often cited source. Oddly enough I believe on his latest build he did not weld anything that way. You do have to take when when designing such a joint, but most of what has been posted on the internet is not flawed.

I dont think that trying to engineer in flex is the right thing to do. I think the designs were engineered to take into account flex of the materials of the time, but flex was not put there on purpose. Look at the claims of todays truck manufacturers. They are talking about the most ridgid frames of all time. When someone designs a rock buggy, there is very little if any flex. In your case, I would put stiffeners on the front part of the frame, then box and stiffen the rear part of the frame. Should leave you with a nice working rig.

Found these using Google. To give you an idea of how the MJ is setup vs. the XJ. Atleast in the rear. I dont know if there was anything special from the cab forward.