A locker or LSD in the rear will make the truck MORE prone to spinning out in a turn. I've had an S15 with LSD and an IH ScoutII with LSD and both had that same problem. With an open diff, you have one wheel spinning and the other anchoring you. With a locker or LSD, both are spinning as is the truck.
Now having said that, I still think an LSD is good for the snow, you just have to get used to what it will do while turning. Never had any trouble on highway curves, just in-town cornering.