Adam, I haven't done Hawaii but have done all the rest except a half dozen in New England. We did Boundary Pk. in May of 2002 and found the road to be in very good shape. We had no problem with a 2-wheel drive van carrying 5 passengers. Except for winter approaches to some of the hp's one shouldn't require 4x4. Winter is obviously a different matter with chain requirements, etc.
Incidentally, as a Washington State climber I've ascended Mt. Rainier 3 times in mid-winter. On one of those (first winter ascent of Ptarmigan Ridge, first winter traverse of the peak) we returned to the Paradise Ranger Station parking lot only to find that we had to probe for the roof of our Ford van. One night during the 5 day trip we received 9 feet of snow, plus lesser amounts at other times and the van was completely gone. Eventually, we obtained a pleasing "clunk" with a probe, shovelled for a couple of hours to uncover it and then waited for the big rotary plow to work its way to our location. Some highpoints are trickier than others.