The best I can come up with is how you tighten your diff.
I never broken a diff screw on my hotbodies or pro4.
It is a steel screw and it streches. If you try to tighten it like a gorilla all at once it weakens the screw and snap shortly with use. If you slowly tighten the screw and build up the tension. .run the car...check for diff slip and adjust as needed you shouldnt have any problems.
People wreck all types of screws at my local track...they just have not learned what I like to call mechanic tight (ie never used any type of torque wrench in their life..lol.)
I dont think it is due to impact as I !@#@# destroyed a wall at full tilt in 19t 2weeks ago and broke a knuckle and that was it.
Jamie