For a while whenever I backed up the wagon, I heard and felt a thud behind the drivers seat. My indy solved the problem this week, after nearly pulling out his hair. There is a bushing that centers the rear part of the drive shaft into the rearend. This bushing was allowing the driveshaft to move slightly upward when in reverse and thus the thud/clunk. He spoke to a Volvo rep and was told that the whole housing needed to be replaced. Being old school he refused that option. A bit of searching and the bushing was located for $59 and not hundreds. No more thud... Hooray. Hope this might help someone with a similar issue.

Jim
2000 V70XC AWD SE