Well...being that they are in a Z28, possibly hard launches with very sticky tires like drag radials or slicks. I would say normally rearends last a long time but if the rearend has been abused.....then any stock component will suffer failure. The engine could have been hopped up, the tranny and the crictical item in the driveline being the rearend is never upgraded so it is the weakest link in the driveline. Just my opinion Three possibilities: too much torque from the engine can cause the ring & pinion gear or spider gear to fail. Inadequate lubrication can cause general failure. Ring and pinion and/or bearings can fail simply because the differential is old and worn out.