I'm not a locker expert, but I've been running an Aussie in the front D30 for most of this year.
I would never recommend an auto-locker for the rear axle. That would essentially mean your rear end is locked at all times.
Running the auto-locker in the front has been working out very well for me. It allows the front to act like an open diff when I'm not in 4x4, but locks up when I switch to 4x4. It helped the JK when going up sandy/muddy hills.
I have been warned, and I'll pass this warning along, that this will RADICALLY change the way the JK drives in snow/slippery conditions. Since the front axle is locked any time I'm in 4x4, if I have to use it on the street, it could result in a situation where the Jeep doesn't want to turn a corner if the road is too slippery.
If you're not going to be driving in snow/ice, then an auto-locker in the front should be sufficient.
Since the cost of an autolocker is far less than a selectable, they are a good way to get into the locked axle world on a budget, just remember the limitations.
(I managed to get in as a test JK for Aussie Locker, so mine was "free")

This winter will let me know if I keep the Aussie, or save up for a selectable. It'll depend on how I feel the JK is in snow, since this is also my daily driver.

 

Detroits front and rear, enough said. Damn near bulletproof and don't have to worry about compressors failing, lines breaking, clutches working, etc. If you are an easy driver, you can drive in the snow with Detroits. You have to get used to the Detroits around corners and KNOW not to mash the pedal, or even touch it, around corners.

 

Grimscale that was perfect, exactly what I was looking for, now from an off roading perspective, would a front locker be sufficient or should you also have one in the rear, now I know the answer will be yes it will be better to have one in the rear but can you get by without it. I am not planning on doing this just trying to learn more about them. Someone was just selling an D30 aussie for like 200 on here yesterday.

 

an aussie in the front would be a GREAT asset to your rig...you most likely have a LSD in the back anyways, the stock Trac-Loc limited slip, coupled with the BLD (brake lock differential, part of the traction control system) in our JK's that could provide you with plenty of extra traction, providing you the ability to get yourself into plenty of trouble out there.
For a tight budget, that's a pretty decent setup...not everyone can afford fancy selectable lockers. As mentioned, if your gonna put one somewhere, put it up front.
Do yourself a favor and google how differentials work, plenty of sites out there that can explain how everything works far better than I, heck...even youtube has videos for you to put it all together.

Keep in mind also that if you have an '07 X minus tow package, you probably only have a d30 up front with 3.21's...if your going to regear in the future, you'll have to toss that front case(carrier) and swap out the lunchbox aussie into a case for 3.73 and higher gear ratios....unless you have an '08 which has 3.73's stock...oh, and if your not confused enough yet, in '09 they went back to 3.21's stock in X's minus tow package...sorry, I just haven't seen what year JK you have

 

here... watch this http://www.youtube.com/watch?v=K4Jhr...eature=related ....it's old, but the PERFECT video to explain how a differential works. Once you understand this priciple all you need to understand is that a 'locker' acts to lock the drive axles together, no differentiation...which locker you chose now...

 

I have an '08 X w/ 3.21's.

 

for real? See...I cant keep up

 

No, but it does mean you are a "wanna be".


Which is not a bad thing, everyone starts with the desire to learn.

 

Loved the video on the differential video! Answered a few questions for me.

 

There's nothing wrong with a non selectable locker. NUTHIN. They put um in vehicles from the factory for years.....AND with bias-ply Firestone Wide Oval tires to boot. Those were some of the hydroplannest tires EVER made. The way some folks talk about non selectable lockers...that combo shouldda killed more people in the 60s and early 70s than all wars combined.....but I don't know of one single lawsuit ever filed due to a locker. The fact is, yes, they do lock the axles together under normal (straight line performance) conditions. Some are more aggressive than others. The Detroit Lockers have this reputation. They are VERY good at their job. If I were to get a locker (and I have one for the front), I would go with the Aussie or something similar....simply because the preload springs in those type lockers are not as stout as what's in the Detroit units, and they will unlock a bit easier....and earlier. You'll see the naysayers say "don't run a locker on the street.....blah blah....." The truth is, if you're careful and get used to it.....AND don't do anything STUPID, you'll be just fine. I have yet to hear or read about anybody gettin killed by a locker. 'Course there's always a first time. I guess you could drop it on your head.