My DIC displayed SERVICE RIDE CONTROL. I know that the rear shock is defect. How do I diagnose the rear shock. When I go over a bump or pot hole it make a clunk sound. This is funny because it been making that sound since 3 years ago but I hardly drive it (less than 300 miles since in got it), and it finally incate that it was bad. I know its a self leveling shock. Any help would be apprecciated.

How do I diagnose the rear suspension.

For the struts, replace them with conventional 'passive' struts. The 'active' ones are around $800 a pop. If you can find them.
As for the 'service ride control' message, go to www.caddyinfo.con (http://www.caddyinfo.con) ,click on 'how to' and scroll down to suspensions. There it'll tell you how to kil the message. Each Caddy is different in how you do this.
I did it to a '92 Seville, and that's simply a software change. The '93 Eldo took the addition of a relay.