Kabooms

I have gone through two differentials in 6 months, both times the differential began whining and it continually gets louder until the pinion gear ends up shedding teeth when doing a pull. Here's the setup:

First Diff-S13 R200 Differential with Nismo 2-way lsd, Motul 90PA fluid, changed twice in 4k miles. Broke at around 5500miles

Second Diff- S13 R200 rebuilt with new OEM bearings and crush sleeve at local shop, same Nismo 2-way differential (cleaned and checked for damage) KAAZ LSD diff fluid.

CAR- S14, 1JZ, R154 Trans, 435whp,370tq. The car has never seen the track but I do occasionally make a few 2nd through 4th gear pulls.

Has anyone had this issue before or know why this is happening? I have read that the R200 is good for 500hp in a track car, and i know others with close to the same hp that do not have any issues.