4x4 (to get the neutral position in the transfer case) for starters, but there are some 4x4 AWD systems that will trip you up even with one of those. It really does depend on the truck/SUV. You need to get into the owners manuals to verify!
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake