Volvo: 99 V70 AWD wagon, o2 sensors, rear differential

Question
I have a 99 V70 AWD wagon and have been hearing and feeling a 'clunk' noise when coming to a stop. It happens when the car is cold or the weather is cool, then gets less noisy and eventually goes away. It sounds and feels like it is coming from the passenger rear area. I had one mechanic tell me it was probably a rotor, but i have recently had my brakes and rotor done and that mechanic said a rotor would not cause that sound- especially if it went away when warm.

The only other thing that is wrong with my car now is that i need to have the O2 sensors replaced (code read at volvo dealer). Does this have anything to do with the noise?

Also can i replace the O2 sensors myself or is it fairly complicated. I did replace my own ABS module, and took care of that problem.

Thanks for your help

Answer
Stephanie,
The noise you hear is not a common problem. Take the car to a shop that works on the Volvos and knows them. Take the tech for a ride to show them the noise. My guess is something is loose or a bushing is bad that causes the clunk. If nothing shows up loose or worn you may have a bad driveshaft coupler, between the shaft and rear differential, or center support bearing. O2 sensors will not cause a clunk. O2 sensors screw into the exhaust and plug in electrically, not real difficult. Make sure that you get factory parts and do both front and rear sensors.