Nice summary, Dave.
My version, shamelessly copied from Dave's:
- A conventional axle with an "open" differential is always 2 wheel drive with a varying ratio of power going to each wheel depending only upon wheel speeds.
- Under the worst circumstances and when you need it the most a standard differential results in your vehicle being limited by the driving wheel with the least traction. Variations of differential design (Posi-trac, Track- loc etc.) and computer controlled traction systems were developed to transfer power to the slower-moving drive wheel (with more traction) under these circumstances. Some "torque vectoring" differential designs can even apply more torque to the faster-turning wheel if that is desired for conditions.
- Modern 4WD or AWD systems distribute power between front and rear axles in many of the same ways as the differential of an axle does between left and right wheels.
- Variables like having the proper tires, driving skill, horsepower, clearance, terrain and traction systems are of comparable or greater importance to the basic features such as 2WD versus 4WD.
- Some 4-wheel drive systems - most often found on pickup trucks, some Jeeps, and more traditional SUVs - have a low range position in the transfer case. Such very low gearing (in the transfer case or otherwise) provides a large advantage in deep mud or loose sand and going down a slippery slope in control.
That performance in mud of that car in the antique video that started the discussion probably had a lot more to do with ground clearance
than anything else. Hopefully you won't be that deep in mud when towing your Escape!