I agree with the other posters - check your ball joints, and possibly think about just replacing them anyway, as they could probably go for a refresh anyway.
But the general consensus is that the shudder is a sign of a worn viscous coupler, and is generally not terribly harmful - though it sounds pretty bad and I'm sure it's not really all that 'good' for the car either :)
You can not rebuild them. Not without some extreme expertise in this area anyway. I've been advised not to let anyone take mine apart by members on various boards/forums.
I did look into getting a replacement from erie-volvo, and they wanted ~$500 for a used one that they warranty for 1 year. Not a bad price actually if it's really bothering you. The labour on installing one of these would be a pretty significant cost if you're not doing it yourself.
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1998 V70 AWD Turbo 190k+
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