Good time to inspect all rear bushings. The main rear bushings at the control arm-to-axle connection is probably bad. These take special tools to install. The trailing arm pivot bushings may be OK, but are getting bad with every mile. The upper control arm bushings are easy to fix. The panhard rod bushing are probably still OK.

I recommend that you use the ipd polyurethane bushings for all simple links, and Volvo OEM parts for the main bushings. You'll have to take the car to a good shop for these to be installed. Make sure the shop knows to support the rear axle assembly well after removing all the linkages. If not, the axle will hang by two very small brake lines.

Getting the bushings installed will do wonders for handling, directional stability, etc.