Its opposite number is cracked:

No wonder it caused a vibration:

If the circlip pliers won't lift the circlip out in one go pull the ends into the groove, the back should lift up, and a thin screwdriver or probe will lift it out:

A bit of a fight to remove the UJs from the yokes but I got there in the end. The two damaged bearings are opposite each other, there is no trace of grease, and one side of the spider has been ground away. All four in good condition at the gearbox end and with plenty of grease, one wonders if nipples are really needed - as long as they are adequately greased on assembly! It must have been heading that way for years if not decades, but with no UJ nipples I'd not been messing with that end of the shaft. Obviously one needs to twist and shake each UJ at each service:

Shaft and flanges cleaned up and painted, new UJs fitted:

Alignment marks correct: