There's quite a few companies that make frame stiffeners. Iron Rock Off Road, RuffStuff Specialties, Hooligan, TNT, etc.
It depends on what you want, full set or just mid frame, but honestly, there's not much difference among the brands. Some are thicker than others, but it also depends on how hard you want to beat on your XJ. They are all basically the same, IMO.
I'm partial to the Iron Rock ones. I just installed a full set on my 99. They fit well and are reasonably priced. I would use them again, for sure.
As far as rockers go, I made my own. I looked into all sorts of rock sliders, but I couldn't find the features I wanted all in one set, so I opted to make them myself. It was a little bit of work, but I'm happy with the results.