The Willy's Wheeler does not come with a locker or a locking rear diff, it comes with a limited slip rear axle. Big difference there.
Well, that sounds more like it. automatic lockers tend to have weird driving characteristics when doing the daily grind. They're typically quite noisy and clunky when used on city streets. You also can't accelerate around a corner with them, because they lock up under load. They work perfectly fine if you know what you're doing, but would be quite disconcerting for somebody who doesn't know how to drive them. It would have been a weird choice for a manufacturer like Chrysler to put them in a vehicle that will likely see 99% of its use on pavement and purchased by non-off-roaders..
I have an Aussie auto-locker ( lunch box ) in the front axle of my Wrangler. It is invisible when driving in 2wd. Once 4wd is engaged it gets the job done. Love it. 
Well yeah, when you're in 2wd there is no power going to it so it doesn't lock. A lunchbox works by locking when torque is applied, then decouples when torque application is removed. Therefore, if you have a locker in the front, when you're driving around in 2wd no torque is being sent to the front diff so the lunchbox locker remains decoupled. The problem arises when you have a lunchbox installed in the rear differential of a rear-drive vehicle (like a JEEP). If you try accelerating around a corner, the applied torque will engage the locker and your tires will squeal, scrub, hop and jump. You CAN drive with a Lunchbox in the rear, but you need to know that you must coast around corners, and only apply power when going straight. This is why I found it suprising that Chrysler would install a lunchbox locker equivalent in the rear axle of one of their products (turns out they didn't, it was just a limited slip) when you consider that 'normal' people who wouldn't know how to drive a lunchbox locker will be driving it.
The article was just wrong, and should have specified that the rear axle was a limited slip axle, rather than an 'automatic locker'.
Just as a reference for those who don't know. You've got four different types of differentials:
Open Differential - each wheel can spin at a different speed, power is sent only to one wheel, always the wheel with the least traction - this is the most common type, mainly due to low cost.
Limited Slip Differential - Each wheel can spin at a different speed, power is distributed between wheels either by mechanical gears or clutch packs. Limited torque can be sent to the wheel with more traction, but not always.
Automatic Locking Differential - When torque is NOT applied, each wheel can spin at a different speed. When torque is applied (i.e. accelerating) the differential 'locks' up and both wheels now spin at the same speed, even if going around a corner. Torque is split evenly between both wheels, regardless of traction. This is an effective but compromised solution, but also usually only costs a few hundred dollars for the part (can be installed in most differentials).
Selectable Locking Differential - Acts as an open differential until you press a button. At that point either compressed air (air locker) or an electric relay (e-locker) will 'lock' the differential so that both wheels spin at the same speed. Torque is split evenly between both wheels, regardless of traction. The JEEP Wrangler Rubicon and Power Wagon have these both front and rear. This is the most effective but also most expensive way of increasing traction.
Modern vehicles like the JEEP Cherokee will use computers and sensors to shift power around from wheel to wheel using a combination of limited slip and/or locking differentials and the brakes (brake the wheel that is spinning to transfer power to the opposite wheel with traction). This is typically more effective than open differentials or even limited slip differentials alone, but usually aren't as capable as full-on lockers.
