According to the Mauna Kea Visitor information center:
2-wheel drive vehicles are not permitted above the Visitor Information Station. A 4-wheel drive vehicle with Low Range is STRONGLY RECOMMENDED. About 300 yards beyond the station, the pavement ends and the next 4 and a half miles are a steep graded-gravel road. You should check with your rental car company to see if you are allowed to travel on Maunakea.
Is it actually difficult to drive on this road with an AWD car in practice?