Update: it was the inner tracta joint that partially seized due to low oil level. Our mistake, obviously. The long drive generated enough heat in the joint to blue the steel. As Richard said, the ensuing galling created high spots on one face and low spots on the mating face. So under load they locked together and prevented the faces sliding. The inner joint only pivots vertically to accommodate suspension movement up and down, so it didn't knock when turning, just when loaded.
The tracta joints look particularly susceptible to low oil level because their small diameter means they loose the oil splash sooner as oil level drops.
Another question: we have no spare left front/right rear bevel box so I need to steal parts from some spare right front/left rear boxes we have. But the bevel gear design in the spares is significantly different. The bevel gear is one piece, incorporating the hub, unlike the original gear which has the hub riveted to the gear. Also the tapered roller bearing between the tracta joint and the bevel gear is much larger than the original. See the photo of a good spare below. Are they interchangeable?
It looks a bit laborious setting up the bearing preloads, lash, and tooth contact without the special tooling in the manual.
Malcolm