How much warning did it give you?

I've got to decide whether to replace the rear axle on my 1992 wagon w/196k. I posted previously about a loud mystery clunk. A reputable Volvo shop says that everything in the rear suspension and drive shaft is tight, the axle bearings are ok and the only other source must be the gear train in the rear axle is shifting when the loading on it changes. They even retorqued everything in the diff and that didn't change anything.

In between clunks, it's pretty quiet. It sings a little at certain speeds over 40 mph when the engine's pulling.

I would like to take a long highway trip. Assuming the shop is correct and this differential has enough internal slop to make a banging noise from hard braking or reversing direction, can I risk taking it cross country? I once had a rear end lock up on a 122, so I know it can happen.

Right now I've got semi-synthetic gear oil in it. I'll switch to full synthetic in the hope of slowing down the decay.

Any help you can give would be greatly appreciated.

Doug Harvey