If you've traveled Tioga Road through Yosemite National Park at the height of summer, you no doubt have been struck by the grandeur of the high-country scenery. And, you've no doubt noticed the car-clogged trailheads that dot the two-lane road. And the crumbling road shoulders. Well, so have Yosemite officials, who are now beginning work on a plan to rehabilitate 27 miles of the road.