Marathon Road and the Pinnacles are easily accesible and can be drove prodimanetly by a 4x2, high speed and mostly smooth. (Given seasonal changes could make it worse)
Billy Goats bluff proves to be the first challenge. It can be rutted and loose rock can make it slippery but a vechicle with moderate clearence and dual range 4x4 should come down it with ease. Coming back up Billy Goats requires more skill and a few sections can be quite technical but those who take there time should make it up without to much hassle.