The reason why the rears would need to be stiffer than the fronts is due to the motion ratio. The original rear spring position is a lever with a ratio of around 2:1, so the 250lb rear springs are actually only 125lb as that's what the wheel see's due to this lever ratio. If it's a coilover rear unit like a set of Ast sportline 2's the motion ratio becomes 1:1 so if that was fitted with a 250lb spring, the wheel would see the full amount.independent shock and spring, like standard