Bruce, most vehicle mfg.'s have online versions of their owner's manuals. It is probably worth you time to look at the 4X4 sections in the manuals of the trucks you are interested in. I looked at the owner's manual for a 2014 Silverado 1500 a few minutes ago. The 2014 Silverado 1500 appears to have a 4X4 unit similar to the one on my 2008 Silverado 1500. The 4X4 on my 1500 was a great system and I think it gives you what you are asking for. You can select 2WD, Auto 4X4, 4 Wheel high, 4 Wheel low, and Neutral.
3/4 ton trucks I have owned are different in that they do not have the Auto 4X4 function.
From the 2014 Silverado 1500 owner's manual:
AUTO (Automatic Four-Wheel
Drive): This setting is ideal for
use when road surface traction
conditions are variable. When
driving the vehicle in AUTO, the
front axle is engaged, and the
vehicle's power is sent to the front
and rear wheels automatically
based on driving conditions. Driving
in this mode results in slightly lower
fuel economy than Two-Wheel
Drive High.