truck and trailer total loss due to black ice

I don't know if it's true in all cases, but I know in many cases AWD means you can't really ever lock the vehicle in 4wd. You have the option on the dial, but you're really only "locking" a clutch system together in the transfer case and with enough force in certain situations it won't hold. I know this is the case for the AWD transfer case in the light duty Ram, for instance. In a standard transfer case you are actually engaging gears.

There are so many systems out there now that the combination of features is nearly endless.

Toyota Land Cruiser, Mercedes 500 and other high end SUV have locking center differentials AND locking axle differentials. Some use air cylinders to activate a dog clutch some use electricity. Certainly, Unimogs and other SERIOUS 4WD vehicles have a number of methods for 4WD. My wife has a Lexus SUV with more ways to manipulate the 4WD system that you can imagine. With both axles in LOCK and the transfer case in LOCK it is nearly unstoppable! But you can't steer! Once you get out of trouble you need to unlock the axles to regain steering control.

My last GMC Yukon had AWD and Center Differential lock 4WD. I loved the AWD function especially up here in NE where we have roads partially covered with snow during most storms.

My current Dodge 2500 Diesel has old fashion, floor shifter, NP205 transfer case. I'm running oversize Nitto Grappler tires and destroyed a rear universal joint by having the truck in 4WD while towing my boat up a launch ramp that I had to make a relatively tight turn on dry pavement at the end of the ramp...BANG! Sounded like a rifle shot! My Yukon was perfect for this as the front wheels stayed on the dry part of the ramp while the rear tires were on the wet, slimy part of the ramp.