All four can spin if the traction is even - which doesn't happen too often. I don't have locking differentials in any of my 4WD trucks and I often see all the wheels spin in my fields - sometimes all four, sometimes just 2 or three and sometimes they change back and forth. And, on any one particular axle - if only one wheel is spinning - there still might be some power being applied to the other. Back when 4WDs weren't all that common - one trick used to be - when one wheel lost traction and was spinning - to put the parking brake on gradually until it equalized the traction and both wheels kicked in. In regard to the Volkswagen 2WD zipping right through where the 4WD truck would not? Happens to me all the time. Our little Subarus (in 2WD mode) go just about anywhere without engaging 4WD. But - my Dodge W250 - a slight grade and damp grass and sometimes all the tires spin an it goes nowhere. I usually have to back up a little onto flat ground , then go forward and get some momentum going. The bad traction in big trucks seems to have a lot to do with how stiff the suspension is. My Dodge W250 has the worst traction and rides the roughest. My Ford F250 rides much softer and gets much better traction. And yeah, tires matter too. But - I've got a few 4WDs that are deathtraps on ice and snow regardless of what tires I have on them. And, to the converse - I can put summer tires on any of my Subaru 4WD wagons and they'll go just about anywhere - and stop just about anywhere.
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