I assume that all the drive train as been cheked to make sure that wishbones, and turnbukle are really mooving free...
I control it regularly removing the up screw of the shocks.
It happens to me that some wishbone shaft where slitely bended.
- If you don't use antiroll bar in the rear you can add it, if you use 1,3mm you can try the 1,5 mm.
- If you prefer you can also use stronger rear diff oil.
If anyway it is still difficult to improve you can chek weight balance on each weels to make sure there is not 30 grams difference...
I hope it will help you...