Contrary to the two previous responses, there has been plenty of discussion around here about spring rates. I don't know how easy it would be to find in a search, but give it a try. Use my user name, it should help, as I'm sure I've responded to every coilover thread. Also, XJ weights are pretty similar, so you should be able to get a good idea of what you should run. Again, contrary to what has already been said, it's not that complicated. If you're going to search Pirate for more info, search in the XJ forum as there have been a few threads there about spring rates.
Do you have a heavy front bumper and winch? Or no winch and a prerunner bumper?
Spring rates on the front of XJ's have been in the 200/300 to 250/300 range. On my buggy I ran 175/300, but my front weight is less than some XJ's, and if I had continued to run coilovers I think I would have upped the spring rate a little. Quite a bit can be done with the transition collar.
Give us a better idea about your build and goals. There's much more to talk about.