If you're never going to leave the pavement, you don't need a lift to fit 31" tires on your Jeep. If you are going to leave the pavement and you want sufficient room to really flex your axles and stuff the tires in the wheelwells, a 2" lift (even a simple budget boost, aka BB) is all you'll need. Here is a budget boosted TJ with longer shocks on 31" MT/Rs showing a stupid amount of flex:
Even if you don't plan to go overboard like I do, I still think a BB is a wise idea if you're running 31" tires. It makes the Jeep look about perfect and it still rides like factory.