[trailerpup]

It is the front differential which is cast iron. That is the center section of the front axle. All of the transfer cases are aluminum. The switch to cast iron for the front differential was made due to the original aluminum housing flexing under "extreme" situation which resulting in the front differential breaking gears in the ring and pinion. If you do not abuse your rig you should have no problem with the aluminum differential housing. The Alpha is rated at higher torque and horsepower, just be sure they upgraded the material. Swapping out the aluminum housing for a cast iron unit is an "upgrade" that some I5 owners have done.