31s fit a stock XJ the same as they fit a stock MJ -- slight rubbing on the LCAs at full steering lock. I have 31s on the '88 XJ (on factory rims) and have had no issues (other than the LCA rubbing, which I don't consider to be an "issue") for two-plus years.
The key is that the 31s have to be on factory Jeep rims. No aftermarket rims have 5-1/4" backspacing like the factory rims, so on aftermarket rims the tires sit farther outboard and hit the flares.
This is true. I have 31's on Jeep Wrangler rims with Spidertrax spacers and a 3" Rusty's lift. Still rub on flares, left more than right. I can only turn halfway then major rubbing. If I take the front flares off I might be alright but I think I'm going to get Bushwackers as I like the stance of the wheels.
But yes, I learned the hard way that non stock wheels on 31's can rub even with a lift and spacers.