Did you ever find a solution to this 747_not_777?
Looking to do the same thing in April
Afraid not SRG. The best option seems to be a rental SUV at the airport to Death Valley (if that, indeed, is your location), and then pick up a dedicated Jeep off-roader in the park from
http://www.farabeesjeeprentals.com/ - you can rent in the park from September through to May.
But if Death Valley's not your intended off-roading destination, then obviously this isn't going to work out too well...
Whatever you do, hope you enjoy.