answering the original question about Diff Lock on the 4Motion and how important it is, well from my experience I've used it and needed it 3 times, each time I found myself in deep fresh snow in my local roads of the Peak District in blizzard conditions going up over the exposed roads. in such conditions you come across snow drifts, and regardless of the fact the snow plough tries to keep it clear, the drifts build up very fast in such conditions.
anyhow, we always have run our 4motion Cali with M&S tyres, and in most conditions that's all that's needed, but i've found us temporarily stuck in drifts on 3 occasions and needed to depend on the diff lock to get us out.
one most memorable time was whilst patiently and carefully following the snow plough from Whaley Bridge over to Macclesfield via the old Highwayman road, on this occasion the plough got stuck fast in a 6 foot drift due to a car being stuck ahead and us all getting quickly swamped with snow as the wind billowed. we all quickly realised we'd have to back out of the situation fast, but the width of the road cut by the plough was only one vehicle wide, and after attempting and failing to turn our Cali around, we realised we'd have to reverse back up the hill. basically this was bloody hard and often at risk of loosing control, but with the Diff lock on we pulled ourselves back up the 500yrd drift and out of the situation. the Plough pulled the stuck car out and we all headed back the way we came.
Without the Diff, we'd all have been stuck fast in that Drift.
It's priceless when needed.