It will take a little custom work, like new spring hangers, but it's been done-- check
www.ifsja.com, or the Jeep forum on Pirate4x4, or even the M715zone.com, which is for the military variant but a lot of people have modified them. FSJ front springs are REALLY short, and you dont' have much room for a longer spring because it will stick out in the front. Skyjacker makes decent lift springs for FSJs, and... Rusty's Offroad(?).