I finally got the video loaded up, you can see the forces applied to one leaf spring with a broken axle tube.....

 

Hope you welded the other tube to the differential housing also, just to prevent any future issues with the same thing happening on the other tube.
I've never preheated the differential, and have not hear anything saying they're cast iron? I have been told they're cast steel, and easy to weld to, so I've welded them without any issue.

 

Yes, I did weld the other one just in case....Steel/Iron I just did what the speed shop told me...lol