Sounds like the upper mount may have snapped. If the coil spring broke your car would look lopsided and the rear fender would be just above your rear tire.

To service this you will need to take the back part of the trunk apart. Both side panels and remove the floor area.
Once that is done you will need to undo the top three bolt nuts for each shock.
Then loosen the lugs on the tires, jack the car up and put it up on jack stands. With the tire out of the way this will help you have better access to be doing the work. Once tire off and car is on up on jacks.
Then loosen the bottom bolt to the shocks. the shock should just drop through the coil spring, watch the upper mount it may snag on the coil spring.
Once that is out, install the new shock and mount in reverse order here.

To answer the question on the IPDUSA Anti Swaybar. The original Volvo unit is a 21mm unit. This was installed for US cars only for a softer ride. Over in Europe they are thicker for the Stiffer ride.

The IPD unit is a 25mm unit. It will stiffen the rear of the car up, giving you better cornering ability and be more stable with a load in the back than the OEM setup. If you do change that out you might as well change out the two sway bar links at the same time.

Any questions, let me know. I can be reached via email at rpiescik@weareoil.com

I have been a volvo owner for almost 25 years and do majority of my own service repairs on my cars.

I hope this helps.