The ride control on your Denali is a computerized system. If the system detects a fault it should display a serivce ride control message or turn on a warning light. Have you seen this? The ride control computer should store a fault code when it turns the light on. You will need a scan-tool to access the computer system a read the fault code(s). After reading the code(s) you will need access to service information for your Denail which will detail what each codes means and how to diagnose it.