No help, just commiseration. My '71 1800e developed a clunk and drivetrain shudder that felt exactly like a bad u-joint. I didn't have a garage at the time and it was winter so I just took it by the Volvo dealer to have them put the new u-joint in. They put it up on a lift, then said I better come look at it.

No bad u-joint. Instead that cast-iron collar had broken all the way around and the OD was more or less just hanging on the tranny output shaft. Luckily no real torque get routed through the OD case when the OD isn't engaged, otherwise it would have spun around and ripped out the wiring and speedo cable.

As it turned out no damage was done to either tranny or OD (luckily I took it in right away) so for a moderate fee they found a collar and installed it.

If yours is cracked it may be on its way to being broken.