So, I've seen a lot of threads asking about this but nothing with more than 1-2 pictures and very little description.
My rear bumper is an AJ's offroad bumper. It was mounted with some 1.5"x2" angle iron that bolted to the bottom of the frame. It had some mounts that hooked with one bolt to the factory bumper bolts. This worked well enough, but the bumper flexed more than a few times and dented the corners of my hatch.
This probably wasn't a valid concern but I worried about pulling a heavy trailer with the relatively small attachment point - the potential to 'pull' the bumper free of the frame mounts or the tongue weight to 'push' it off the mounts or to bend them. I knew they flexed at least a little because the dents in my hatch.
I set out to cut off these mounts, open the frame, stick 2x4x3/16" in the frame and weld the bumper to those 2x4's. I drilled holes in the 2x4 using the factory holes as a template and welded nuts inside the 2x4's. I also installed HD offroad shackle relocation brackets at the same time.
The 2x4's ended up being right at 5 1/8" sticking out of the frame, and 16" into the frame - so ~21 1/8". The ends are cut at an angle because that was how I bought them and wanted to pick up as much as I could inside the frame. There isn't really room to go longer than 16" because the frame starts to turn in.
My mounts are as follows, from the bottom up. 1/4" steel (HD Offroad brackets) welded to the frame. The frame, 3/16" box 2x4 with nuts welded inside. I intend to drill holes through the side of the frame and weld more nuts inside the 2x4's to pick up additional attachment points. My math shows 18" of weld attachment for each 2x4 to the bumper. Up and down each side of the 2x4, across the bottom, and then up the sides 3" where it attaches to the underside of the top of the bumper. 2" across the top.
I was able to pick up the factory hitch bolts and another a few inches forward, so there are four per side instead of three like I had. I think the back of my Jeep would part with itself before this bumper would part from the 2x4's, and the 2x4's from the frame.
Now, for what you came to see! The pictures:
Bumper almost out of the frame:
Here is the back of the XJ with the bumper out. You can see that I cut the back of the frame open in this one
Passenger side frame hole:
You can see that I left the shackle bolts long. I drilled the 2x4's to match this bolt so the bumper would be easy to line up, and provide an additional semi-attachment. About 8" in the stock frame on the passenger side you will find a tab of 3/16" steel welded in. It spans the width and is about 1.5" deep. It's welded front and back and was difficult to chisel out. Some guys just cut this section out, some notch the bumper mount. YMMV.
Here's the obstacle on the driver's side. The gas tank filler tubes pass through the frame and are protected by this metal sleeve. I considered removing the sleeve and filler tube and cutting a hole for the gas tank filler tubes to pass through the bumper. It seemed way over kill so I notched the 2x4 instead.
Here's an overall of the mounts
The other side:
Obligatory weld shot:
These welds were made by a friend using my Hobart Handler 140 with .030 flux core. No beveling before welding.
Here's a shot of the notch out in the driver's side 2x4.
Getting painted in my make shift paint booth:
When the paint dries I'll put it back on and get some of it mounted for you guys.