xj axles on an mj

will xj axels fit on my mj, will this achieve a spring over setup?

yes and yes. You might have to relocate the spring perches though....i'm not sure if they are the same width on MJs and XJs, but i would think they are.

XJ rear perches are wider than MJ (ask me how I know this) the springs will splay out enough, but I don't recommend it.
Also, the shock locations are different keep the SUA spring/shock mount(from the MJ), and shave off the XJ shock mounts.
Another issue, is the load biasing valve for the rear brakes on the MJ. Pull it out, and replace the proportioning valve with an XJ unit.
You will gain 5 inches of lift by going to a SOA, get a set of 5 inch springs for the front to level out the ride.

Yes. I have XJ axles under my MJ. The front is a direct swap. You will have to move the rear spring perches about an inch. You probably want to move them anyway to get the right pinion angle. You'll also want to move the shock mounts. I cut off the old MJ shock mounts and cut them so that the shock bolt was even with the center of the axle. This worked well with the 4.5" lift shocks. I bought a 4.5" lift kit that include all the front lift pieces and just the rear shocks. When I was done, I was sitting at 5" over stock both front and rear. The spring over in the rear actually gave me 5.5", but the original springs have sagged a little over time. The rear drive shaft will be extended further, but I found it didn't need to be lengthened. I still have the load balancing valve in the back, but have it wired in the pull up position. It's a pain to get it properly bled, btw. I've also since converted to Crown Vic Disk brakes in the rear.