I plan to purchase a new tow vehicle within the next two years. At the present time, I am towing a 3500 lb travel trailer with a 2006 Honda Pilot 4WD. The 4WD is Honda's name; actually, it is AWD wit...
GMs have had an auto 4wd function for several years now. The '09 Tahoe I'm driving has it. I don't use it however. I leave it in 2wd and shift to 4wd if I decide too. Jeep probably has the closest thing to what you're looking for. I've always just shifted to 4wd when needed. I think most any modern 4x4 is "shift on the fly" up to about 45 mph.