Below is a cross section of the rear axle and the assembly of the pinion shaft.
I think what went wrong is number 10 in the assembly instructions.
This is from the GMC 47 maintenance manual; they used the same rear end.
Attachments
Document_2024-07-24_074017 (2).jpg (329.81 KB, 239 downloads)
pinion 1946.jpg (257.83 KB, 247 downloads)