& there was me thinking rear shox are one of the very few easy jobs on a KJ.
XJ top mounts are completer different, as are the (well chronicled) problems.
This problem is more akin to leaf spring front mounting bolts where Loctite is more of problem than rust - &, if you're working at ground level poor access is another factor.
Douse with penetrant of choice, heat bolt head 'til cherry red, that should deal with the Loctite. More penetrant, a rattle gun will come in handy, although you'll probably have to "crack" the bolt with lots of leverage first. Tip: don't try to work within the fender well. Use an extension, supported on a jackstand, so you can concentrate effort on that 2'+ breaker bar, now presented at a very convenient working height.
If it comes to it, drill a small hole close to where the bolt enters the mounting, insert straw from penetrant spray, so you can soak the nut/tip of the bolt. It's a still a bit hit & miss, so just apply far more than you think necessary. However, that's only going to help if rust really is the problem.