Locked Or Loaded
As far as off-road performance goes, there's nothing you can do to your Jeep that will make as big a difference as adding a rear locker. And once you add a front locker, you'll be amazed at the tractor-like clawing abilities your ride will possess. Lockers help offset the drawbacks of a stiff suspension because forward momentum is no longer impeded if you lift a tire. Lockers add finesse to your driving because you'll be able to idle over obstacles rather than take a running start and making a hail-Mary pass through difficult lines. Lockers have drawbacks. There, we said it.
When a vehicle makes a turn, the outside tire needs to spin faster than the inside tire. An open differential allows the axleshafts to spin at different speeds, but always sends power to the tire with the least traction. The drawback is if you lift a tire, that's the side that's going to spin. An automatic locker locks the axleshafts together when under drivetrain power. The axleshafts can only disengage and spin independently when the vehicle is coasting. The downside is when turning under power the inside tire will chirp and bark and the outside tire will scrub. Also, an auto locker will instantly engage or disengage when power is applied or removed, which can cause noticeable jerking, banging, and clanging. A manual locker allows you to engage or disengage the locking function of the differential at will. This allows quiet and seamless on-road drivability without giving up any off-road performance. The downside is high costs and sometimes complicated methods of engagement/disengagement, such as bulky cables or air lines. Here's the staff's take on the subject.
I've run almost every locker ever made in a variety of vehicles ranging from big and leaf-sprung to small and coil sprung. When it comes to lockers, I've got some very definite opinions-but no set rule. In other words, my choice very much depends on my application.
For starters, let's talk front lockers. If my vehicle has locking hubs I'm gonna go straight for an auto locker. I like their simplicity and the fact I don't need to remember to turn it on or off when on the trail. The only downside is if you bust a shaft under power there's a higher-than-average chance you'll damage an automatic locker, so, I'll factor axle strength and tire size into the equation. I normally go for a Detroit Locker and, although they can increase turning radius, I've found I can turn sharply by modulating the throttle during slow, slinky trail work. On the other hand, if I'm running a vehicle with unit bearings or no locking hubs, I'll go straight for a selectable locker. I've never had a metallurgical failure with a pneumatic locker, but I have had my fair share of leaky seals, crimped air lines, and other associated troubles. It's been enough to steer me over towards the electric locker camp. I much prefer simply running a couple wires through the diff than routing a bulky cable or plumbing an air system.
In the rear, I let my wheelbase and suspension determine my locker selection. For a long-wheelbase vehicle with leaf springs like a J-truck pickup or even a Cherokee I'm more than happy to run an auto locker. I never even noticed the Detroit Locker in the rear of my M-715. On the other hand, running a Detroit Locker or automatic lunchbox locker in the rear of a short-wheelbase YJ or TJ Wrangler was enough to make my hair white. It was so bad I wouldn't let my wife or friends drive those vehicles, so I wound up switching over to selectable lockers. For the sake of stability and predictability, I'd really recommend a selectable locker in any short-wheelbase Wrangler, CJ, or vehicle of that ilk. That is, if you're not manly enough to run a spool. But that's a different story.