diff plates
 

Why are the diff plates made out of two plates and welded together instead of being pressed out of one piece?

purely for the fact that most people don't have the facility to bend 5mm plate accurately. if you do then go for it.

I would also add, if an incorrect grade of steel plate was used there would be quite a high probability of an un-noticed stress fracture within the area of such a tight 90 degree cold formed bend in 5mm plate ;)