According to PhysX documentation shock absorbers are already implemented and sprung mass is taken into account as well:
http://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/Manual/Vehicles.html
search for PxVehicleSuspensionData section]
Perhaps it requires more tuning to get it right. Unfortunately they don’t explain a complete algorithm there. Might need to look into code for this.