I have ridden parts of the State Park on my motorcycle. There are some nice jeep roads in there. The best scenery is on the West End towards Fresno Canyon.
The National Park has a few more sites to see in the back country. River Road East and West (in the park) are great.
Black Gap Road is one of the best, never maintained.
Have to agree on 170 as a great road. Late Winter, early spring is the best time.