my car shimming when I go on rough spots in road, especially wet streets, why? on 2005 Volvo XC70

I never had problem until new rear differicial installed 3 months ago. When street is wet it is bad if I go over a bump or a railroad pass in the road. The rear will seem to slid to right then left.

Asked by for the 2005 Volvo XC70
If it only has happened since rear end work was done I would have the rear suspension blots and bushings rechecked to see if anything is loose or worn, check the condition of the tires and tire pressures.