My recently purchased 2008 Volvo XC90 AWD (almost 18,000 miles and has the CPO warranty) has had a vibration throughout the whole car when taking the foot off the gas petal and coasting. It also happens sometimes when I brake. The faster I am going, the harder the shake is, and it seems to be worse on the highway. Its in any kind of weather (dry, rain, snow) and recently seems to be getting worse. I took my car to the Volvo maintenece dept. and they said they could not duplicate the problem but noticed my steering fluid was low. They filled the steering fluid, but the problem still persists. Driving to work this morning, my car was shaking so bad (when I took my foot off the gas pedal, not when accelerating) that my sons rear facing carseat was bouncing off of the back of the passanger seat. Its frustrating and now Im believeing its a safety issue when my kids are in the car. What could this problem be?

I was thinking maybe its the traction control? Our tires (pirelli)? Motor mounts?... I HAVE NO IDEA! And clearly my svc. dept. doesnt know either!

 

It could possibly be the the wheel bearings. I would highly suggest taking it back to service and going on the road with them when they test drive it.
The other possibility is that the emergency brake may be seized causing the shaking....