Rough Country or Rusty's Offroad, both decent kits at a great price.
When I ran a 3" lift on our old '98 XJ I pieced it together myself on a budget. 3" coils from Rusty's Offroad for $100, a 2" block kit for the rear from JC Whitney, about $40, and a pair of lift shackles for the rear.
31"s may rub a little with a 3" kit, we ran 30" tires and didn't have a problem. XJ's have the tightest fenderwells going.