Most I have seen broken is where the pin goes thru the axle itself.
One side or the other usually break of from around the pin.
It can be made stronger before the break by adding a 1/2 plate top and botton.
Both of the axles on our 1680's have been broken. they brake right where the tube is welded on to the main frame of the axel. I agree a plate on the top and bottom is the way to go, Just like the axles on the 2388's. I dont know if the welds can be made with it in place. It realy wouldent be that hard to remove from the machine. When my axle broke I was in the worst possible spot. Hard ground a pick and a shovel a few blocks and 2 bottle jacks makes for a good time.
They can break anywhere. They crack out around the pivot tube and can also just fail and snap in two there. Or they can break the extension accross the bolt holes. The wider you run the axle, and the wider your tires, the more strain'n stress you put on it. I got tired of allways chasing cracks and jacking the machine up during prime running time, so I made my own.