506000
First of all, the laws will vary state to state. You will need to check your local laws. If you're really curious, most states have an on-line version of their laws. A little time on the net will find them. Most of these have a search engine capability, so you can look up the information you're after for your particular state.
Having said that, I'd think that few states would require anything more than what the jeep came with. I.E., for a jeep that came from the factory with fabric doors, I don't see a requirement for steel doors to be put on. They would be "grandfathered in" in most cases.
Now then, I wouldn't be surprised if the Feds regulated the MANUFACTURE of vehicles without doors at some point; however, that's different from whether you're allowed to drive with them off. Also, the manufacturers may have simply put doors on in response to (1) customer demand, and (2) lawsuits.
Remember that riding a motorcycle is still legal. That's a whole lot more dangerous to the rider than riding in a jeep without doors. This is a different issue from riding with the windshield down, since that affects your abilities as a driver, while no doors simply affects your chances of surviving a T-Bone crash.
Hope this helps.
