Regarding the warranty, the only thing it would "void" are broken parts or wear as the result of the part or the installation. If your trans goes out, it's not because of the locker and they cannot void your warranty on the trans (abuse is another issue). If your rear brakes go out (leaks, etc), they cannot void your warranty. But if the gears start making noise, or you break an axle, it's always possible they will give you a hassle (and possibly rightly so), because Ford didn't screw up the installation, your shop did, and lockers can be hard on axles. They will still probably give you a hassle if for instance your trans went out but it would be hard to prove. Chances are, they would not even know about the aftermarket LS unless you told them about it.
Anyways, I'd give it a try this winter and see what happens. I've never had a problem with 2WD and street tires in snow / ice. Just careful driving. I'd fully believe that with 4WD and good factory tires, you will be fine. But then again, there are people who have trouble staying out of the ditch on sunny days.......