These newer ones never really run in 100% 2WD. There's always a little bit of power sent to the front axle. When there's more demand for traction, the 4WD computer will give more power to the front wheels, then back off once there's less slippage on the rear wheels. It's not an instantaneous reduction in 4WD either...Once the system detects slipping severe enough to engage the transfer case fully just once, it goes into an "intelligent" mode, where it looks at throttle input and engages the transfer case if you go beyond a certain throttle percentage. It will gradually back off on this as it sees the rear wheels are able to provide sufficient traction, but until it does, you get some engagement of 4WD on dry pavement if you're on the gas hard enough.
I did discover a way to reset it though...At normal road speeds, let off the throttle, press 4x4 HI, then back to 4x4 Auto, and it will reset.
Mine drives me nuts when it does this...I have a gravel driveway at work, and a slight little spin in the gravel will get the 4x4 engaged, then it's in "intelligent" mode halfway home on dry pavement, and I can't give it more than about 1/3 throttle or the 4WD is engaging.
