Tapered vs straight diff axles

FWIW... I raced Opels with all of the taper in near to the diff and they could twist the thin portion of the axle between the taper end and the diff splines. That design concentrates the stress in the thin portion of the axle in a short distance, and actually raises the stress level. A longer thin portion of the axle will lower the stress in the thin part.

But that is just general design info. A longer thin part will be less stable and may bow or 'corkscrew' up easier under severe torque, so take the above with a grain of salt. This may be why Opel pushed the axle taper in close to the diff.

On your axles, is the one with the step near to the diff a tapered step or a square step? The latter is obviously more prone to breaking.