Maybe put some stuff like soft wax glued to the inside of the fender lips. Then measure the space between the tire and wax. Then take a lap or 2. Then measure the gap to see how much wax is rubbed off. That should tell you how much the rearend is moving to each side.
Or maybe have someone with a good movie/vid camera that can stand on each side of the course, and can take a vid of the rear tires as you go around the corners. Then watch the vid to see if you can tell how far the car's rearend/fender lips is moving side to side in relation to the tires/rearend. A good enough camera so you can watch it in really slow motion and still have a sharp picture.
From the 2 pictures in your last reply, I'd say the rearend is moving 2" side to side, maybe.
Go Pro or XD mini cam....
My rear sway bar is disconnected so that is what looks like is flopping around back there. But you can see the distance the sway bar arm separates from the dog bone link. Seems like about 3/8" to 1/2"
