Heavy Duty vs Z71

Does anyone out there know what is different on - Z71 suspension components vs heavy duty suspension components? Seems like the majority of the short box versions of the Ext. Cab 4x4's Z71's in the northeast ride fairly level front to back. I have a heavy duty model which looks like it wants to get under the pavement. It has been that way since new. Can it be changed over, or is it just not worth the $$?