ml320 1999 poor handling on rear
 

Hi I have a merc ml320 1999 with 71k I got a local garage to replace the rear shocks about six months ago with geniune mercedes ones and about two months ago when y went over a bump the back end jumps about to much and goes thud if this makes sense and its a bit twitchy driving down the road now. When you go round a corner it leans alot more then it did before as well. I have spoke to mercedes and they stated that they dont mind examing the rear shocks if i take them off (more expensive) but it would the first faulty set they have seen if thats the case. Any more ideas out there, any advice etc would be grateful.

Ball joints? Bushings?

Its had front ball joints replaced and the rears were fine when they changed the rear shocks so didnt replace them. Could the fact i havent replace the fronts cause an instability on the rear