I took the plow off today, and I found out what the real problem was! It does not make any sense to me, as that part of the pin only has pressure on it when it is pulled to turn the blade.
My theory as to why it was fine when I parked it, and broke after it sat in the garage is that water got into a crack there, froze and split it apart. As it thawed out the water got deeper, and froze again... making the crack bigger until it broke. On the broken part of the pin you can see the different colors from cracks that formed at different times. I am not sure how it cracked the first time though; like I said, all pressure is forward that part of the pin.