On ice and snow, the spool and locker are probably the worst
choices for highway driving because if there is any wheel
spin, it is always both wheels spinning at the same time.
That means that neither of that axle's wheels can help
control the vehicle's direction. With a limited slip or
open diff, generally only one wheel will spin, allowing the
other to help track the vehicle in a straight line. A manual
locker would also be a good choice for snow/ice-covered
roads because you can choose how it should operate. Having
said that, there are many people who drive their automatic
locker-equipped vehicles on icy roads on a regular basis. It
all depends on your comfort level.