No movement at all. I'm pretty sure the factory rails would come off before the rack does.

I've never taken it off, but I doubt it's compressed the rails. As you can see in the pic, there's a rubber coating on the clamps that grips the rail. You tighten it down with a couple of bolts. Trust me, it's solid.
I use mine mostly for camping/road trips, so it usually has a tent, a bunch firewood, etc up there. Occassional trips to the Home Depot, etc.

Not too much weight, but this thing is tough. The weak point is probably the side rails on the KJ.
I got mine from National4WD up here in Canada. Garvin are US based though, so you're better off looking locally.
One thing to be aware of: it's pretty tall. 4" sides, plus a inch or so for the mount. With a 2.5" lift, plus the rack I just fit in my garage.