To clarify further, while registrants must be TLCA members, there is no prohibition or requirement as to what vehicle is used for the event. For example, a registrant has a bone stock Taco and is a TLCA member, but they don't want to scratch it up on the trail. But their trail rig is a Jeep, a Blazer, a Scout or whatever. They are more than welcome to wheel whatever they want at the event, as long as they are a TLCA member and the rig meets safety requirements.
On the subject of one registration for two different vehicles, we do have the requirement that every vehicle that runs trails during the event must pass safety inspection. Also, during inspection, vehicles are also rated according to how they are equipped. This establishes which trails they are allowed to run. Tire size and differential lockers are the main criteria. Ultimately, each trail leader has the final say in deciding whether a given rig is properly equipped to run a particular trail.