I lived in wisconsin for awhile and I used a ford equal-loc which is now an old style limited slip which preloads the clutches with a belleville spring that is not real strong. However, the Ford posi unit can be adjusted while on the bench by lowering the preload with thinner shims. Break-away low limit 50 ft lbs, mud and snow 75 ft- lbs, street racing 100 ft-lbs, and drag strip 125-150 ft-lbs. I would try around 50 ft-lbs. If you are that concerned about ice. Most cities salt the streets and ice is not a "long term " problem. But maybe it is where you live. (However. don't be suprised one wheel in mud and one with good traction break-away might be experienced causing you to lightly apply the brake to load the clutches using the spider gears )
That's the cheep route! They make differentials with dash control to lock and unlock the rear axle. Detroit also makes what they call a soft lock. My friends don't like a locker because as locking operates it tends to drift the vehicle left and right on dry pavement.
good luck!
PS Personnaly, I wouldn't be too concerned about ice because you have very little control anyway.