Question time.

Repair, rebuild, replace?

Hi guys, I hope this is the right place to post this. I have some questions for the brain's trust.

1995 3.0l turbo diesel auto 2WD. Ykzh100gmfpqt
Located Melbourne VIC

Back story.
Purchased her a bit over a year ago, fixed her up partially thanks to this forum. Got her back on the road and then drove her all the way to Exmouth to see the sun disappear and then drove back home. Just over 10,000km all up. It was a wonderful adventure and I've been driving her ever since.

She has some things that need fixing some rust, windshield will probably need replacing soon but nothing major. Or so I thought.

It was a scorching hot day and I was on my way back home after going to an event in Geelong, I pulled into a servo to get a cold drink she was driving fine up until this point, no odd sounds, nothing out of the ordinary I can say this with certainty because it was such a hot day I was paying much more attention to everything.
Anyway she put her foot down quite suddenly and refused to reverse. Putting pressure on the accelerator did nothing it was like she was still in park. Sometimes she would reverse for a second but then lock up again immediately.
I managed to coax her out of the parking lot by cycling through gears with a lot of grumbling and shuddering coming from the back. It was like the brakes were on so I optimistically thought it might be the rear brakes, old soft lines and all that.
Took her to my mechanic, not the brakes. He thought it was the centre diff. As luck would have it I managed to find a replacement diff from a 2WD surf, lot of research and comparing part numbers later it was exactly what I needed. But it turns out that my vans diff is perfectly fine.

Now the going theory is it's the transmission.
But the specialist that my mechanic talked to about it was apparently stumped and said it's a very strange series of events. At the moment she is with my mechanic and some days she will reverse like nothing's wrong other days she locks up.
I've been quoted in the area of $4000 for a rebuild and that's not including anything else. For context this is around the price I bought her for and I did most of the work to get her roadworthy because I couldn't afford much more than that. In doing so I learnt that I enjoyed giant mechanical puzzles and it wasn't as scary as I thought, well until it's my only vehicle I guess.

So I think it might be the torque converter but I'm not a professional by any means and at the moment, we don't know for sure exactly what the problem is and I've been told even if it is the torque converter the whole transmission is probably bad.


So I guess after all of that my questions are

Has anyone experienced this before?

Does anyone know what I should expect from a repair (cost and life expectancy) if that's even possible?

Does anyone know how much a diagnosis would cost?

Is $4000 for a rebuild in the normal range?

Is there a cut off year for a replacement transmission? I read somewhere that the transmission changed around 1997 and made later models incompatible.

That's not even getting into finding a replacement.

Are there other transmissions I should or could be looking for?

Any advice appreciated, feeling a bit stuck at the moment.

Thanks for reading