Here’s the answer: You can use either “2WD” or “Auto” all the time. If you don’t have any need for extra traction, using “2WD” may save a tiny bit of money on fuel and possibly some wear and tear on the four-wheel-drive components.
“Auto” in your truck operates in two-wheel drive by default. And, when it senses a loss of traction, it automatically adds power to the other wheels. So, “Auto” is safe to use all the time, and that would be my default mode.
“4WD Low” and “4WD High” are not safe to use on dry roads. They’re only for while you’re actually driving on slippery terrain. Those two modes lock your center differential to give you maximum traction for when you’re stuck or in snow, sand or mud.
The problem is that when the center differential is locked, the wheels aren’t able to turn at different speeds.