X2 what Wayne said - actually, I normally leave my hubs locked all year unless I head out onto the highway in the summer..me, i lock the hubs first of nov and leave them locked till the end of feb or even march. you lose about 2 mpg in the real cold for the resistance of turning the front diff but the peace of mind is worth it to me.
vehicles with an auto locker in the rear should have front hubs locked in winter months, PERIOD. that ass end can come around even in conditions that you think it shouldn't.
But then I live in a rural area.
And I push that 4x4 button, or pull the lever at any speed. Just be absolutely certain no tires are slipping when if you do it.