The only 4x4 trucks I ever drive are rentals, so I'm certainly not an expert. But I do know that if the driving lanes are snow covered, I'll throw it into 4x4. The way I see it, that way each tire is sending 1/2 of the tourque to the ground (vs 2wd) and is 1/2 as likely to slip.
Obvioulsy, I'm not terribly concerned about wear and tear on these trucks, as I'm driving them for a week at most.
Something to think about, if the road is bad enough to use 4wd, is it good enough to go faster than 80 kmh?