Driving in 4 wheel drive in a straight line on dry pavement/rock will not hurt the truck. The damage is done when making turns on dry pavement/rock. When the vehicle makes a turn, the front axle rotates at a different speed than the rear axle. This causes the "jumping" or "bucking" sensation (if you've ever made a tight turn in 4 wheel drive on dry pavement you know what I'm talking about) and can lead to drivetrain damage.
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You've got city hands, Mr Hooper . . . you've been countin' money all your life.