I removed my rear bumper, removed the rear crossmember, built a rear bumper out of 3.5" x 3.5" square tubing, mounted it to my frame rails with 1/4"x2x4" angle iron with 4 grade 8 bolts and welds on each side. I ran up the frame rails about 12" with the angle iron to make it as sturdy as I could because what your doing is extending the frame. Then I removed the fuel tank and installed a fuel cell in the bed, because when you move the axle back, the pumpkin will hit the tank unless you slide it back also.
Then you have to remove the spring hangers and move them back, I mounted my rear ones on the bumper, hence all the work on the bumper. Remember that all the weight will be riding on the bumper, so it needs to be an extension of the frame. All thats left is the driveshaft, the e-brake and regular brake lines need to be extended. Oh yeah, gotta move the shock mounts and bump stops. Also the front spring hangers will only go back about 3-4" before they get to the upswing in the frame, so if you go farther than that you have to build a support (or wedge) to level them.
I ended up with 7.5" more wheelbase but it'll take a while to do it, and I wouldn't suggest it for a daily driver, but for a hardcore offroad rig it's great. Oh yeah, you also gotsta cut out them rear wheel openings. And it's all got to be straight, so measure, measure, measure before you weld!
Whole lotta work, but it's worth it.
1974 CJ-5 (Rockcrawler)
2001 Grand Cherokee Laredo (Curbcrawler)