I'm about to start doing mine, I hope. I have the replacement rocker panels. Before you take this on, you should take a look at the rocker panels now on the Jeep, to see what that single piece of metal consists of. It's quite a piece. As they come, they would require a great deal of weld separation and rewelding, so if you do get a set, expect to cut them to fit a smaller area, unless you want to do a great deal of body work including undercutting areas of the the body that overlap the rockers. I'll probably have to take off the front fenders, as well as the rear flares, because the rust goes right to the end of the rocker at both ends, but hope to cut along the top of the panels where they are hidden by the door, and do the welding there, rather than chisel the things out from underneath the door pillars.
If you look at an XJ carefully, you'll see what I mean, and also why I keep putting the job off! If the rust is minor (mine isn't), I think it might well be easier to weld in individual patches than to replace the whole thing. If there's still a lot of sound metal, one possibility would be to cut the rusty portions out, then tack weld or rivet patches behind the original, followed by filling with bondo. That often works better than trying to patch over the top.