8.75 help!!!!

Ok my brain is smoking and I am at a total loss....

I purchased Nitro axles shafts with Green style bearings for my A body 8.75 rear end with a clutch type limited slip diff... I have 2 741 cases with a 3.91 clutch type diff one which was previously installed and driving with a howl but fully functional.

I purchased the second 741 case built the same way used but functional. I installed the newly purchased diff with the factory style axles and bearings to load the car on a trailer without any issue. I relocate the car and install the new axles with the green bearings which required me to remove the thrust bearing from the noted diff.

After installing the new axles with the green bearings I slowly drove the car in reverse 10 feet and heard a loud pop. At this point I had not movement as the driveshaft would spin but the differential would not so I assumed either the pinion popped or the axles did not fully engage the diff. I pulled the diff, swapped in the old diff and again the new axles shafts... Same results. Back up and a pop resulting in no movement though being driven by the driveshaft. When pulled the diff, no visible damage to either the axles or the diff.

I measured both axle sets of
shafts and they measure the same length so I am at a loss here.

What am I missing?