Going out solo in any outdoors activity is about really knowing your limits, meaning both you and equipment, and having proper self-support gear. I'm not an experienced off-roader by any stretch but I do frequent solo mountain biking and hiking/backbacking. With the caveat of my given lack of specific experience, I'd say you'd want recovery gear that doesn't require other people: shovel, traction aids, winch and associated kit, etc. 4x4 being a critical part of that; as well as SHTF survival equipment like bivy sack, mylar sheet, extra food and water, reliable communications (cell phone if you can trust the service, ham radio, sat phone, etc), possibly a personal locator beacon. It's always a good idea to have an itinerary and someone who knows your route and timeline who is expecting heartbeat check-ins. While it's great to prepare for the worst, you can avoid hairy situations by sticking to routes you know you have the capability to handle and ones that are well-trafficked. For example, when I'm solo mountain biking I ride much differently than when I'm out with riding buddies.
Ultimately the best piece of gear to pack is common sense.