does your jeep have the stock dana 35 rear axle(c-clips...tiny axles...weak housing...just plain bad)?
have the gears been changed and what are they?
how wide are your rims? because 15x10s stick out a bit further than 15x8s giving the tire a lot more leverage on the axle.
there are a lot of variables, but...
you could safely run 33" tires as long as you are easy on the throttle when off road. your acceleration will **** more than it does with 31"s. top speed would theoretically be higher but your 4cyl will prolly never get you there. changing from 30"s to 33"s on a jeep with 3.73s and a 5 speed transmision will make 5th gear completely useless. changing your gears(depending on what they are now) to around 4.10:1s will help the acceleration a lot. do not go any lower(higher numerically) than that because the pinion gear is way too small even with 4.10s to give the owner much confidence. there are complete dana 35 upgrades available that come with ring and pinion, locker, stronger axles, and a housing truss that keep up with 33" tires fine, but, they are very expensive. plus you'll have to do the front axle to match.
performance upgrades like a header with a free flowing exhast and k&n air filter may help some too.
