So I have had 4X2 with Posi, and got stuck in a flat grass field. Before replacing that truck, daughter was with a friend with a new 96 F350 4X4 towing a four horse gooseneck pulled into a farm yard nose down hill and a slight turn, and started to spin the tires in 4X2, shifted to 4X4 no issue.
That was the last 4X2 I owned, coming home from the Oregon Coast January of 2011, snow falling hard, the first climb into the coast range is about 5% to 6% grade about 1/3 of the way up holding 45, rear end broke loose, dropped into 4X4 end of story kept on climbing without an issues the rest of the way home.