1999 Seville SLS Service Stability System

When I go over a bump in the road the service stability systems comes on the dash, the abs lights up and the traction control light comes on. I notice the car sitting low in the back and don''t hear the air shocks  pump.  When I turn the car off an on it resets until I hit another bump.  I did take it to cadillac once but it was very intermittent then, now it always happens.  I know it had 3 codes in the history but I do not remember what they were.    I think I have a bad shock,  how do I tell which one is bad.  Also where can I buy replacement shocks at a reasonable price.   The next time it happens I can take it to auto zone and have them give me the codes.  Please advise on how I should proceed. 
 
Thanks

I just had the computer scanned and the following codes are showing during the failure, C1223, C1227,C1234.  I am going to replace the left rear hub assembly. 

My rear shocks are leaking as soon as I go over a bump the service stability system comes on. If I put 160lbs of salt in the trunk the service stability comes on.   I guess I have to replace the rear shocks.  But i may have more probles as the abs comes on and traction control  I had the Rear BCM scanned and once it had some left rear speed sensor codes. I don''t know what to do , any suggestions.  

I had the car scaned  today here are the codes, can somone help me out with this one.  Where are the grounds that I can check.
What can you reccommend that I do.
C1223 - Left Rear Wheel Speed Sensor Input Signal is 0
C1234 - Left Rear Wheel Speed Circuit Open or Shorted
C1227 - Left Rear Excessive Wheel Speed Variation
C1242 - Pump Motor Circuit Open
C1284 - Lateral Accelerometer Sensor Bias Malfunction
C1291 - Open Brake Lamp Switch Contacts During Deceleration