Or the splines are others have suggested. I had problems over a decade ago and I am remember more bits and pieces as I think about this thread.
I know I went from a B14 SE-R VLSD to a peg leg when I cracked my case and I am sure something else happened inside the transmission. I never took apart that trans, a friend of mine did and replaced stuff with other parts. I know I got a different case, my gears were ok, and I recall him mention that he did replace my VLSD unit with a lower mileage unit he had from his original B13. I know my axles were completely fine because I still have those exact same axles in my B14 now.