You can get an idea of the lift by measuring from a flat surface to the bottom of the Jeep underneath the door. With stock size tires it should be 18". If you are correct with the amount of lift you have you should be able to run 31" tire with no problems.
I have a 2" lift kit and I got about 3" of lift out of it. On the street its fine but if I were to really go off road it would be hitting the flairs for sure.