Back on track now. The center differential does not use a traditional vicious coupling unit, but instead another clutch pack similar to the ones used by the auto tranny itself. There is a duty solonid that controls hydrualic pressure to this pack forcing power to the rear, or lowering pressure allowing it to stay up front. This is what I know of the system and I think it uses the ABS wheel speed sensors to detect the slippage. On the other hand I think WOT and powermode will override any programing and forces the power to the rear for that sporty like take off. I will ask a technician for sure. Is Eddycat2000 on here yet? I think he knows for sure.
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Well, I'll try a post here. The ABS sensors have nothing to do with the transmission operation. If you've ever seen one of these transmissions apart you can tell that the front diff cannot be disabled, (unless it's broken). There is no center diff, it's called a transfer clutch assy.
I tried to make a drawing and it came out terrible, so here it is in my (probably poor) best description.
Power is inputted at the torque convertor. Inside of the extension housing there is a set of reduction gears. Those gears redirect power to the front diff. So, you will always have 100% of the drive train power to the front diff. What varies is the transfer clutch pack that is located on the top reduction gear. A solenoid activates line pressure as needed, determined by the TCU and the speed sensors to lock the rear output at certain ratios of RWD. You will *never* only have RWD unless you have a messed up transmission.
BTW- power mode only changes the shift points in the trans, it does not effect AWD perfomance. Neither does the manual button on your shifter, that just forces the trans to start in a higher gear for less wheel spin on take off.
I hope this was clear, and that it helps resolve the question.