Very interested in this process Julian. I too lost 3rd and 4th two nights ago and limped home in 2nd with OD.
I pulled the top cover and expected to see a broken shifting fork, but it looks fine. The issue appears to be limited travel of the ring in the center of the syncro hub for 3rd and 4th. (I can't get 3rd or 4th and when I was trying to shift, there was strong resistance in the gearshift lever, but not a "clunk" metal to metal resistance. It was more of a binding feel.)
I'm thinking my clips may have broken as well. I've not looked in to see yet, but hoping that is the issue if it can be repaired without pulling the whole unit out. The trans is quiet and has never given me any issues, so it doesn't need a rebuild. I just need to be able to shift!
I know nothing about transmissions, so I'm in uncharted waters here... Any additional comments would be appreciated.
Thanks!