I want to do some things in the Waimea Canyon area that the Kauai Revealed guidebook says require 4WD. Is it 4WD that's needed, or is it ground clearance (making a 2WD SUV OK). I have a 4WD reserved w/ Avis, but an SUV (presumably 2WD?) was cheaper. Neither Hertz nor Thrifty have a specific 4WD category (only SUV, unspecified).