If I'm not mistaken, the ES9000's are nitrogen charged:
Summit racing's decription:
Nitrogen gas-charged Explorer Pro Comp ES9000 shocks are designed for increased load capacity, as well as high-performance ride and handling characteristics.
ES3000's description:
The expanded, gas-cell-charged, twin-tube design of Explorer Pro Comp ES3000 shocks increases fluid capacity by 50 percent, to reduce internal operating temperatures while protecting vital components.