The point of a back protector is to protect your spine, kidneys and ribs from hard blows. The point of a hard shell is to disperse the force of impact, much like the shell of your helmet.
Too bad the US is too backwards to have any kind of standards for back protectors. Luckily, the European Union (EU) has a standard for motorcycle armor, called the "CE" standard. You can buy CE rated back protectors in the States, and they are recommended. Some jackets come with (take your pick: flimsy foam/dual-density foam/EVA) that won't protect you nearly as well.
Make sure the back protector covers more than just your spine- remember it's for the ribs and kidneys, too. Also, consider wearing a chest protector as well.
Knox is a good company for armor. They're not cheap, but neither is missing 3 weeks of work because of broken ribs.
If the gear you currently have doesn't have good armor, different types of armor can be purchased from New Enough and other gear dealers.