OK, the diff can be described as a bunch of bits that work together inside the housing of an axle. In our case, we have a 3rd member that carries the ring and pinion gears, bearings, and the "differential" which in our case is a limited slip differential. Locked, selectable, Limited slip, and open are the terms used to describe what type of diff or carrier an axle assembly has. Being that we have an LSD in the rear means the ring gear bolts directly to it. If you were to order an ARB (model RD85), it would replace the LSD,and the ring gear would then bolt to the ARB instead. This means you would have to shim and adjust your gears so that they mesh correctly again. If you were to order a "lunchbox" style locker (Aussie Locker) you would also need to find an open carrier/diff to use in your stock axle housing. You end up swapping out the spider gears in that carrier and you replace them with the locker instead. If you already had a 3rd member set up with the 4.30 gear ratio AND an open carrier you would not have to adjust the gears to mesh correctly unless you just didn't trust them as is. The point is you didn't have to remove the ring gear or the carrier from the assembly to use the locker. So you can spend $400 on an Aussie Locker and another whatever for an open carrier and have all that swapped into your axle housing. Or you can spend $800+ on an ARB and have the same adjustments performed for the same amount and then spend some extra change for switches and compressor. Disclaimer: Just my understanding!