From what I have read and seen underneath the vehicle, there is no center differential on the expedition. If it had one that it would be AWD not 4wd. The computer uses the transfer case to engage/disengage the front axle. I don't know where you came up wit this but if you give me a good reference than I might believe it but trucks do not have center differentials.

Mark

Here is the definition of ControlTrac 4 wheel drive.

The "Auto" mode normally routes all engine torque to the rear differential until wheel slippage is detected by electronic sensors. When slippage is detected, an electromagnetic clutch activates in the transfer case, progressively transferring torque to the front differential. This mode should be used all the time on the road and in nearly all weather conditions. The "4H" mode locks the center clutch, forcing the front and rear drive shafts to turn at the same speed. This mode should only be used in relatively extreme situations such as off-roading or deep snow or sand. Since the center clutch is locked, turning can cause binding in the drivetrain which can cause damage. "4L" is similar to 4H, but additional low gearing is utilized to maximize torque, such as towing a boat out of water. Due to the low gearing, the fastest one can go in 4L is roughly 20 miles per hour (32 km/h). Since 2007, the Expedition's Control Track system has had a fourth position on the rotary ****, 2H (2 high) for more economical driving.