Abother one bites the dust! 
Have you tested pla-abs mechanical prop?
Neater are Strong enough to take the Pounding you Need a Ring of Nylon to act as a strengthener
I don’t see a crack on the parts, I think the problem is related with heat. As the heat rise the parts wear out since abs is not heat resistant. Have you tried filling oil to the differential body? This would reduce the heat by lowering friction. If you see any improvement, you can additionally use something like alumunium sheet between the differential casing’s two parts and a heat sink attached to the aluminium sheet to dissipate heat. I saw you have used nylon also which should be more heat resistant but you would lose strength with nylon.
and It broke right were the Screw ended look how much ground up plastic there is there… Or just print out a Nylon Diff that will solve the problem too also try white grease for Bearings.