My '88 560SEC has a self-leveling rear suspension. I would like to replace the self-leveling shocks with normal ones like on my '86 300SDL. To me the normal shocks give a better ride, are more reliable and cheaper to replace. Is it an easy upgrade (or downgrade)or possible(costwise)?

This would be a "down-grade" for your 560!! Is the system giving you a problem? It's possible to install springs & shocks from a 420SEL, but the value of your car goes down.