because the front and rear driveshafts are turning at the same time....so, all 4 wheels are turning at the same speed and rotation...when you turn, each tire is taking a shorter or longer path in the turn. that causes bindage on paved roads....the tires stick to the road so the drivetrain binds.
on gravel/dirt/mud...i.e. slick stuff, the tires are allowed to slip and slide and still turn at the same speed, so there is no binding in the drivetrain.
you want the same gear ratio in the front and rear....or have no more than 10% difference i believe.... 4.09s/4.10s, 3.54s/3.55s .....etc..
there are some full time tcases in the older trucks, and some newer trucks are starting to do this....an example, dodge dakotas/durangos can be equipped with the NP231(off the top of my head), which is a full time tcase. more importantly, these fulltime tcases are NOT
single speed like a lot of these yuppy SUVs that only see paved roads.
a down side to full time 4wd is lower mpg, and possibly faster/uneven tire wear.
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