A Tru-Trac or Wavelock diff is probably your best option.

One thing I like about a spool is predictability, you always know exactly how it will act because it can only do one thing and that is keep the tires locked together. A locker is a spool, or its not, and the times it decides to come in and out of the locked status don't always make sense.

In the rain I am fully aware of how my Barracuda acts with a spool, its really no big deal because I know exactly what it will do which is usually nothing out of the ordinary. My Jeep has a locker and it's not always predictable, when it locks and unlocks usually is but how the vehicle responds is not.