This is normal for the LS; when your foot is on the brake, if you shift from R to D5, the car will squat, and if you go from D5 to R, it will lift. It happens to some extent in all slushboxed vehicles, but the LS's anti-lift rear suspenion exacerbates the effect.
If you think about it, when you load up the driveline, it's trying to rotate the drive wheels forward, but the brakes are absorbing the forces, and the brakes are attached to the suspension. As a result, when the driveline tries to rotate the wheels, it also imparts these forces onto the suspension, which also tries to rotate forward.
In most vehicles you don't notice it because the suspension mounts on the chassis are both fore and aft of the drive axle, so the forces are balanced, with an upward force behind the axle and a downward force in front of it. The LS's suspension chassis mounting points are all ahead of the rear axle, so there's nothing to counteract the downward force ahead of the axle, and the car squats.
The same is true when you go to reverse, but the direction of the forces are reversed, so the car lifts instead of squatting.
