You could have a shock leaking air. Willwren posted about spraying soapy water on the connection to check for a leak. It was for the struts on the previous generation Bonneville'* but would work for yours too. I'm not sure if that would cause your wear problem, because I believe the OEM shocks are regular shocks with air assist. Perhaps both parts of the shock is bad?
Another thing to check is the shock mount inside the trunk. The shaft for the shock on my passenger side pushed right through it. I didn't notice a change in height, but it did make quite a clunking noise.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.