1998 v70xc driveline noise

I have a 98 V70XC AWD with 76,000 miles.

Between 58 and 61 mph, with a slight pressure on the accelerator,
whether in 3rd or 4th gear (automatic), there is a noise which sounds
very much like, in RWD terms, a slightly tired ring and pinion.

If you take the foot off the accelerator, the sound goes away.
If you go faster or slower, the sound goes away.

My Volvo dealer shop manager went with me for a ride Tues and verified
the sound. Yesterday the propeller shaft was replaced because the
bearings for loose/worn. However the sound did not go away.

I called another shop today and described the symptoms. The response was
that they too had replaced propeller shafts with the same symptoms.

Today I drove the car with the service manager and the shop manager
[front and rear right side seats]. Both heard the noise and verified the
conditions but had no further diagnosis. They planned to test drive a
couple or so used V70XCs to see if they had similar sounds.

Any similar experiences or suggestions about treatment/diagnosis?

k hamilton
my volvo history: s122, 145, 245, 745, v70xc awd