is it common to replace the rear shock absorbers only on 99 buick park ave

no. not on any vehicle. it throws off steering. makes handling at speed unstable.

A thorough suspension inspection should be performed to determine if you can replace just the rear shocks or other components in combination with the shocks. We have seen the rear shocks (specifically the air shocks) fail at a faster rate then the front struts. We have replaced just the rear shocks several times after inspecting the suspension, with no side effects in ride or safety.