I'd guess a broken center differential.

 

Transfer case or tranny

 

The center differential is in the transmission. Having all four in the air is not a very good test. All four wheels should all start turning once the viscous coupling kicks in. But, if the car has ever been towed on two wheels the viscous coupling could be toast. Then, with the car up, all rotation could go to whichever end of the vehicle has the least drivetrain friction. On pavement the front would still have drive.

 

My IX is acting a little finicky as well. Under hard load, especially from 1st to 2nd, I get a slight "slip" when I shift. Only sometimes, and if I mash the gas faster it sometimes does not slip at all. Could my clutch be galzed and or flywheel marred and I'm just hitting the sweet spot every now and then? I have a new OEM clutch with 5k on it. I can't say if it was broken in properly becuase I bough from a dealer who installed it and I can only guess what clowns test drove it.