S15 helical diff grinding

[gpeele]

this is a s15 helical diff installed in a spare s13 non abs pumpkin with the correct output shafts and brand new shims, bearings and seals. all installation was done by a shop sometime ago but the shop has since closed.

just today i had the pumpkin swaped out with my stock one and it was making a loud grinding noise, like loud enough to be heard when the rear is jacked up and the wheels spin freely when the accelerator is pressed. when on the road the noise is much louder and reverbs through the car. i cant take it back to the original shop for obvious reasons, so what could be the problem, bad shims, crappy fluid (75w90 gear oil is in there now). are helical lsds themselves known to go bad? any help will be appreciated.

thanks.