Something is broke in your 4WD unit.  Replace it.

Better yet, junk the truck and buy a GM truck.  You wont have half the
problems.

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to please me, you must tweeze me.

If you are on pavement when this happens, it's the drivetrain complaining.

Your truck is not designed for 4WD operation in places where the tires can
grip the ground. What you are feeling is, differences in tire speed being
transmitted through the drivetrain.

In a turn, all tires turn at different speeds, the left and right are
different from each other, and the front and rear are different. The
differential is able to handle the differences from left to right, but the
transfer case can not deal with the differences from front to rear. The
result is that the truck bucks a little as the drivetrain binds and releases
(builds and releases stress from the tires going different speeds).