OK I finally got around to doing this experiment and with both the transmission and the transfer case in neutral (i.e., no wheels driven, no connection side to side in the front and no connection front to back) the front tires still squealed.

SO - with AWD out of the picture, I'm left with the conclusion that it's just an Ackerman angle thing - and I'll gladly put up with a little tire squealing in exchange for the tight turning radius!