It's for on-road use. I'm not much of a believer in global warming, but the last few winters in the UK have had several snowy periods, which wasn't as regular before, and the councils aren't gritting all the roads (suits me fine - I don't like salt on my chassis). Unfortunately, the crap standard of winter driving in the UK, and the fact that most of the prats with high performance summer tyres don't use winter tyres for the cold season means that as sensibly as you drive on the slippery stuff here, you're going to stop because of other traffic, and getting going again is the trick. I normally use 235/85 BFG AT KOs, which work really well for me, though I recently replaced them with 205/80 Hankook ATs, which also coped admirably with the recent ice (the BFGs wore out and I had the Hankooks laying about spare - I'll go back to the BFGs or maybe General Grabber AT2s when I finish rebuilding the vehicle). From what I have experienced, the ETC does well enough at the back by itself, so I hadn't planned to do anything with the rear, but the front doesn't have ETC and so can't even mount the kerb when it gets really icy. That's why I was contemplating the ATB in the front only.
I have never used any kind of lockable or LS axle diffs, so I'm gratefuul for the feedback of your experiences.