The Whiteline 02-07 WRX & 04-07 STI rear subframe lock bolts will remove the slop from the rear end. The WRX and STI have a rear subframe that is held to the car using soft OE bushings that allow movement. This excessive movement reduces the effectiveness of sway bars, lateral links, and control arms. This inexpensive kit from Whiteline threads bolts into holes that already exist from the factory. The bolts install simply with hand tools and will stiffen up the rear end of your 02-07 WRX or 04-07 STI.
Part Number: KSB750
I prefer the one piece design of these as opposed to others that have a plastic piece around a slightly smaller bolt. The whole idea of these is to make something that previously allowed movement to be solid. These do a great job of exactly that.