For ease of install and smooth ride you can't really beat the Wrangler spring swap with a traction bar in the back.
I installed a 4 link in the back of my SJ and have been more then happy with it. It handles much better on the road and soaks up bumps at speed without issues. If I was going to spend the time to build a suspension I would recommend going straight for coils.
A samurai will never be smooth on rough surfaces though, just not enough wheelbase to soak up the bumps.