Neither above comment addresses the previous identified rear axle bump steer.
Thrust angle is an alignment problem. Rear toe on a straight axle is a bent axle. Both problems are not bump related.
Rear bump steer on OEM leaf spring setups is an intentional or acceptable chassis design compromise solution, and only corrected if unneeded or undesired by modifying the suspension, and not changed by simple alignment changes.
Actually, rear bump steer is caused by changes in the thrust angle as the axle moves up and down more on one side than the other. Nature of how a leaf sprung setup works. Longer front spring length mitigates it to a point, less fore and aft movement as the spring goes thru it's arc. This is where a properly setup 4 link can come into play.
