Pinion shafts and input shafts on power dividers breaking like glass was not that uncommon in the 50's 60's and even the 70's. We blamed it on metal fatigue. When we examined the break, part of the break looked fresh for part of it and old on part of it. Seems like it was always Eaton rears, not Rockwell or Timken, their predecessor,that broke. Quite of things like that broke in the "good ole days" but parts were much more available then.