[truckster]

A locker has the potential to make your sliding problem worse. The reason you're sliding is a loss of grip, not necessarily rotational force. In other words, because the rear of the truck is so light compared to the front, and the drive wheels are pushing the weight, it wants to go in the easiest direction, which may be sideways. Having both wheels spin can make that worse, because right now you have one wheel that's spinning and one wheel that still wants to go straight on the rear. When both wheels are spinning they'll both go in the easiest direction.

If you're going to install a locker, you're going to have to address weight distribution as well. Adding some weight to the rear of your truck will help. And I suspect you'll quickly find that it's going to take a very light touch on the throttle in icy or snow conditions to keep going straight.