Highway 12 between Yakima [1] and Mt. Rainier [2] traverses 4,500-foot White Pass. The route climbs westward along the Tieton River [3] through heavily forested areas with a few clear views of Rainier along the way.
During most winters, the large White Pass elk population and a small herd of bighorn sheep are fed at the Oak Creek Wildlife Recreation Area (two miles west of the intersection of Highways 410 and 12, and 17 miles west of Yakima, 509/575-2740).
Best times to see them are early morning or late afternoon, when the animals are most active.
Links:
[1] http://www.moon.com/destinations/washington/columbia-gorge-and-wine-country/yakima-valley
[2] http://www.moon.com/destinations/washington/south-cascades/mount-rainier-national-park
[3] http://www.moon.com/destinations/washington/south-cascades/white-pass-scenic-byway/white-pass/tieton-river-rafting