I have a samurai and I am looking at getting a mini spool. I have heard that the mini spool breaks stock axles. Its not a dailey driver and it gets towed to the off road park. Most of my offroading is mud and very little rocks. Would i have to worry about snapping axles?

A fun thread comparing the options.  http://www.zukiworld.com/forum/index.php?topic=24758.0

I have run a mini spool since the beginning and have only gone through one set of rear axles in 5 years with 33x12.5" tires.  I haven't taken my current ones out to look at them recently though, we'll see how well they hold up with the new rear suspension.  If you end up toasting a couple sets you can get Heavy Duty rear axles that should be up to the task.

ANY locker will make axle breakage more likely. In a open diff, the power goes to the tire with the least traction. So - when one tire has very little traction, and another tire has great traction, the low traction tire spins (zero stress on axle with traction). With a locker - power will still go to the axle with great traction... so, if you're applying axle-breaking torque & weight to that axle... SNAP.

A selectable locker is the most gentle, but it costs $1,200. An auto locker will allow the free-tire to spin a little - then lock - creating shock load. Shock load = more likely to break. A spool is always locked, so loads tend to be more transitional (instead of shock loads). That's not to say auto locker = exploded parts.

So, ranked by "safe" for your axles:
* Open diffs
* Selectable locker
* Spool
* Auto Locker

There's way more pros and cons to consider before buying a locker - check out the link above. For what it's worth, I'd recommend a spool for your rear (cheap, reliable, and reasonably safe for the axles), and auto locker for the front (easier to steer). Unless you can afford selectable lockers.