I've just come back from 10days there in June (on Kauai and Oahu). I wanted to do the same. But it all got too hard. Jeep jks and ford mustangs are the two mist popular vehicles from hire companies. I got a jk from Alamo (no idea on cost as it was pre booked). At xmas it will be busy, id book now or else you will miss out.
I tried to ride shotgun with some guys from ohana trails (Google it) but I was only on Oahu on weekdays so didn't work out.
I am going back june 2013, I will try harder to go on a wheeling trip somewhere.
Heres me and the jk we hired at kauai.