Are you sure it's not just tyre wall deflection, as in the photo on page 2. If the rim is moving laterally inside the tyre, due to soft sidewalls, insufficient air pressure, etc, the car will seem to be slaloming from the back by itself. I had this happen with some 245/515x13 slicks on 7 inch rims, if the tyre prssure was too low. Looking at the 2 outside rear tyres on Harrys' cars, in both instances the rim is pretty much in line with the side wall of the tyre. As the tyre you use seem to be a Hoosier, I've found them to be weak in the sidewalls ( a mate uses them on his hillclimb openwheeler) & crossply's. It might just need a few more PSI to make a difference. It's also a bit cheaper to change than fabricating major suspension bits.