This shot is of an ARB selectable locking differential - air activated, it works both as an "open" differential and a fully locked differential. you can see the air tube in this shot.
Rather than say - do you need Lockers, lets back that up - and let's talk types of traction devises. They lump into three categories and get to the end means different ways:
Open differential:
A common term for a Standard differential which will contain your gear set of a particular ratio, and a "Carrier" that will hold the ring gear, ends of the axles and have a set of 4 gears inside the carrier. These rears can rotate on each other so your vehicle can go smoothly around a corner like the one pictured below. this gives you the best road manners but if traction is lost on one wheel those 4 gears in the middle will spin and not allow a transfer of power to the other wheel, leaving you with "one-wheel-peal" the tire that loosing traction will just spin either on the ground or if off road in the air.
Todays electronic cars with traction control and antilock brake system can trick these differentials to work a little better by selectively applying the brake to the spinning wheel via sensors on the wheel end. this will brake the spinning wheel and force traction to the other wheel - but it's a hit and miss deal and if you're into off road you will figure out pretty quick what all the "locker" talk is about as your buddies drive around you or spend a lot of their time pulling you out. (buy a winch).
Posi traction:
There are two versions below both from Yukon Gear and Axle
Pictured above are two different posi-traction differentials as you can see they look quite different from the open differential - there is more "stuff" in the middle. On the left is a clutch style limited slip. Yukon gear calls this there Duragrip. There are a series of plates at each end of the case and the big wave spring in the middle applies pressure to those plates so when one wheel slips - the springs and plates press outwards reducing slip and transferring power to the wheel not spinning. When driving around corners they plates can slide on each other allowing smooth corning while still providing more equal traction to both wheels. The other one pictured does a similar operation but there are no clutch packs but a series of gears that wind into each other doing the same thing. Both of these provide more equal traction but are still designed to "slip" to allow comfortable cornering on the street. inevitably they can still allow a tire to spin when say off the ground off road - but at a far reduced rate. A trick drivers learn when wheeling limited slip differentials is to slightly apply the emergency brake. this will increase the pressure to the plates and gears adding valuable traction to both wheels - works for the rear only of course.
Lockers.
Below is a Eaton E-locker. True locking differentials can also be split into three segments. Spools, Mechanical lockers, and selectable lockers. The E-locker is a selectable version.
I have used every locker currently available and a few no longer available.
Mechanical lockers are ones that are always "on" they can provide wheel slip - but when power is applied, they will instantly lock together giving even power to both wheels they are connected to. There are inexpensive versions typically called luck box lockers, and more HD versions the first of which was called the Detroit Locker. A trick to cornering with these types of lockers for smoothest operation is to " coast" corners rather than power around them, but its not uncommon to hear some tire squeal and the occasional bang from the differential or a ratcheting sound from luck box lockers on the street, Off road they provide 100%power to both rear wheels.
Spools and really only for aggressive off-road vehicles and drag cars, there are a solid piece and make both rear tires turn at the same speed all the time.
Selectable lockers offer the best of both worlds - though at the most cost. When not active they operate as an open differential giving you the best possible street manners. When activated they skip right to the spool stage for maximum traction. These come in a few flavours and today, almost every truck and SUV manufacturer offers these in their vehicles alongside open differentials and posi traction differentials. Typically attached to their most extreme off-road packages. Of course, the aftermarket has been well represented well before the factory car manufacturers got involved for you to retrofit your rig. The main operation comes from air, electricity or manual cable activation. All work as expected by allowing the differential to act open when not engaged then fully engaged when power is applied.
What works for you really depends on your type of off roading and your budget as you can go from a few hundred dollars for a lunch box locker to a couple thousand for a selectable version plus install costs, typically taking place when re gearing to compensate for those big tires you all want! talk to the group you wheel with, or if thinking of a new vehicle purchase look to those already equipped factory vehicles - rather than pick for colour and those fancy heated seats if you really want to go out and wheel! as they say - lockers (and winch) before light bars!
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