Yes its for the rear and it's a mechanical locker that kicks in when it senses wheel spin. You will be fine with the all time 4wheel drive, I believe 07JeepXK used this locker in his QT-II XK in conjunction with a locked front diff for a DIY QD-II setup, checkout his thread called "powertrax no-slip locker" my tablet won't let me copy the link right now, but its on the similar threads box at the bottom of this page; he also has info in his garage thread. I was gunna put it in my QT-I V6 XK since it was all time 4hi only, but ended up trading the XK in for a Nissan Frontier due to continuing electrical gremlins. Install is really easy, just slightly pull out each axle shaft enough to mount the 5 components, then reinsert shafts/reattach diff cover + add oil and your good to go.