I have used nickel rods to weld diffs together, I also weld bolts in between the spiders to make sure it holds nice and strong. It has work on local speedway cars and a few of my trucks, never broke a single one.
Nickel rods are the best choice if the parts are cast iron, which I think they are. In a pinch, Stainless rods are a good choice to weld cast iron because of the higher nickel content than in mild steel electrodes.
