The Road to Backdoor is a dirt road that leads around the mountain and runs directly into Backdoor. This road isn't very popular since Backdoor is such a difficult trail. The road is comprised of mostly hard-packed dirt, with some soft sand as you get closer to the Backdoor obstacle. All 4x4's can easily handle this road.
Johnson Valley OHV area is an open OHV area, meaning you can drive anywhere, and you don't have to stay on the path if you choose not to. Portions of Johnson Valley are shared with the Twentynine Palms military base, and closures go into effect when training takes place. Please be sure to visit the BLM website: https://www.blm.gov/visit/johnson-valley-ohv-area to see closure schedules.