Thanks everyone for the replies.
We have found one California company that will rent to us, however, they have restrictions on the states we can travel in/through. They have two bases, one in Florida (that allow you only to drive in Florida) and another in LA, that allow you to drive no further east than Texas. This completely cuts out New York which is a definite must see.
The reason we want to see so much so soon, is because my girlfriend and I would like to look into moving to America. We want to see a lot, and don't want to go on a group tour, because what we want to see and what general tourists want to see are different.
A bronco is a nice idea, we have a 74 F100 and I love it, but they're far too thirsty to go cross country!