I passed right by the mention in the above post about it mounting on the ctr section. It needs to be as long as possible to be as effective as possible.
Also, what exactly do you mean the car has 'side movement' ? Is it blatantly visible that there is movement between the rear end and the body ? Or is the whole car pulling to one side ?
If you can push the car side to side (when it's static) and there is obvious movement between the rear end and the body, then you have some issues to solve. Worn/lose spring end bushings, etc. But if there is NO movement, you have OTHER things to look at.