If one is using all solid normal hardness urethane bushings in a rear triangulated 4 link, and not driving only on the drag strip, the system by design has no real compliance to overcome its natural designed in bind issues. Urethane would likely fail in due time in those conditions. Rubber anywhere in the links would reduce that bind by allowing some compliance, IMO. The OP implied to me the whole bushing system had failed, and was urethane, if I understood the situation correctly.