Yeah, the factory posi's are weak, the side and spider gear teeth are real fragile.
Care to share how you got 31 spline axles into a 7.5"?? No one makes them, or a posi or spool to fit them, and that size axle won't even fit through the carrier bearings on each end of the differential for that matter.
Biggest axles that will fit through the bearing are 28 spline, up from the factory original in your G-body of just 26 spline....but even then, with 28 spline axles the axles aren't the problem....you get this down into the high 12's at the strip and it's going to start peeling ring and pinion teeth off(pinion stem flexes due to small size and gear mesh goes all to crap and crunch
), and there ain't no heavy duty fix for that except an entire rear end swap to something that will handle the power....12-bolt, 8.5" 10-bolt, Ford 9", custom Dana 60.
There's a reason GM put the 8.5" in turbo Buick G-body's, looks like you're about to find out why with the new engine