I think they started installing the Xtra-Lite in about 2000 in various models and most ended up with them prior to going to the Edge chassis. I'm not so sure that the Xtra-Lite and the Edge suspension aren't all that different with just more travel in the Edge setup.
I would suggest the X-Lite so that you aren't getting it too jacked up in the rear like an Edge setup might do. This doesn't seem like a big deal, but the pre-set caster in the trailing arms makes the thing steer funny. With the shallower rear end with the X-Lite, it should be a nice upgrade without getting crazy.
I put an Xtra-10 in a '94 XLT with a 136 x 1.75. It hooked ok, but steered badly and gave no ski lift when playing around. I then road a 2000 700 RMK with the Xtra-Lite and laughed all day as fun as that sled was to lift the front end up over things under power.