All MD series differential carriers are derived from specialty cast iron.  They are 100% machined and multi-phase heat treated.  This carrier is designed specifically to use Ford's 9"  spider gear sections, which are VERY strong !   The spider gear set is what allows one wheel to turn faster then the other when going around corners (unlike a quad that has a solid rear axle).  Spider gears relieve the stress in the differential associated with going around corners, and it also allows your car to handle better. 

Strong spider gears are important because they can take lots of abuse.  Examples of this are:

1.  Using the turning brake while accelerating (sometimes needed).
2.  Landing from a jump while on the gas when one rear wheel hits a split second before the other (very abusive, a mistake often made by novices).
3.  Burnouts, or dumping the clutch too fast when one rear wheel is on blacktop, cement, or hard dirt while the other wheel spins in loose sand or dirt.

All MD transaxles have 4 spider gears (superdiff arrangement), as seen in our disassembly picture.