I think ford did offer some of their broncos back in the 70s with full time 4wd and a differential in the transfer case. I have no idea how they managed the problem of power going to the axle of least resistance though. I don't even know if fluid coupling transfer case differentials even existed back then. I know in the 80s, toyota offered some of their cars with awd and used a manual locking center differential instead. Much better system, but it requires driver intervention and thats probably why they stopped offering it.

Driver's could be liable to damage the car if incorrectly used, and people today expect cars to think for them. Can't say I blame them considering how much they charge these days for something that still burns just as much fuel and does the same thing.

 

The aztek we have uses a driveshaft to he rear that is always spinning. When the front wheels slip it sends a signal to a pump in the rear that engages the clutches and then the rear tires will pull. It seems to work ok but not what I like.

 

i think the ford escape and a few other smaller suvs have that type of set up too