Weak springs alone cannot cause this kind of tire wear. The more likely cause is worn-out rear wheel bearings. If you take a minute to look, you can see how the rear suspension pivots allow the wheel to move straight up and down, without any angular deflection. The wheel always stays perpendicular to the ground, no matter the load. The wheel bearings are about the only thing that will cause deflections of the wheels like you seem to be describing. It could also be bad pivot bushings, but let's not go there!

Evan Mac Donald

1984 Wolfburg
1985 GL 7 Pass.
1991 Carat Weekender