Re: Joshua Tree Geology Tour Road with 2WD ?
We did Geology Tour Road in the spring. They recommend 4x4 but I don't remember anything requiring 4x4 about the drive. Clearance is more help than 4x4. There are some sandy sections and a couple of mild off-camber parts but nothing challenging comes to mind. I'm pretty sure I've done it a couple times in the past without a 4x4 vehicle.
I've driven a lot of the regular dirt roads throughout the park - you shouldn't have a problem. There are a bunch of 4x4 off-road roads in the park that I haven't driven - the park service tends to give very cautious advice/warnings before driving those which makes it difficult to know whether they truly are difficult or just an overabundance of caution on NPS's part - but I know others on this forum have driven them.
Other than that see/hike as much of the park as possible. It is a great place - one of my favorites. Lunar landscapes, lots of rock scrambling, hiking, changing deserts, vistas. Even though a lot of people visit it still feels less crowded than other places. And even if the weather turns "bad" you might be treated to snow in the desert, which is amazing.
Have a great time.
KeVan: '06 E350 4x4 V8 "Transformer"