Check to see if the tie rod end recall has been done on your XC90:
I remember one owner noted that her drive shaft was shot. It could be...
- Drive shaft/axle
- Wheel bearing/hub unit
- Engine mount
- Loose pax seat mountings
If you can do so safely, visually inspect for drive shaft, steering & suspension components for signs of damage (rubbing marks, etc...) and try shaking each component for looseness. However, I would do this with someone who knows how to jack up a SUV safely.
As I am not privy to the XC90s, this is all I can do for now. If you did not receive other replies in the next few days, try posting a thread in general Volvo Chat.
I wish you the best of luck,