Sure could be the driveshaft center support bearing. Usually this tends to "sing" when cold, but quiets down when it or the weather are warmer. It's relatively easy to drop the driveshaft and cenetr bearing and spin it to listen for bearing noise.

The Volvo differentials are about as sturdy as the Rock of Gibralter.

If you find you actually do need a rear end, shop the boneyards for a low-mileage used one of the same ratio as yours and with the electronic speedo sensor. Because they almost never fail, there should be plenty available for pocket change.

It's unlikely your diff is dead.
--
Don Foster (near Cape Cod, MA)