Gearbox is out of car, on the bench. Trans is in gear, and I can tighten down on the rear drive pinion castle nut but it never tightens. It goes on until I can see the hole for the split pin, but then something "lets go" and the whole spline spins. No bang or pop or anything scary sounding, it just starts spinning.

The gearbox is out because I had a problem last year where the car would not move when in gear. I pulled the trans to find the bellhousing full of oil, so assumed the problem was leakage and ruined clutch, so that's what I dealt with. I replaced the primary pinion oil seat and refurbed the parking brake and all linkage and seals. I did not separate any of the cases. Putting it all back together and after bolting on the parking brake and pushing on the rear drive flange and nut as described above, that's where I'm at.

Any thoughts from those that are more familiar with the inner workings of the gearbox?