In a large bowl, dissolve yeast in warm water. Add the milk, sugar, shortening, salt, eggs and 3-1/2 cups flour; beat until smooth. Stir in enough remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 45 minutes.
Punch dough down. Turn onto a lightly floured surface; divide in half. Roll each into a 14x9-in. rectangle. Spread 1/3 cup peanut butter to within 1/2 in. of edges.
Roll up, jelly-roll style, starting with a long side; pinch seam to seal. Cut each into 12 slices. Place, cut side down, in two greased 13x9-in. baking pans. Cover and let rise until doubled, about 1 hour.
Bake at 375° for 20-25 minutes or until lightly brown. Combine frosting ingredients until smooth; frost warm rolls. Cool in pans on wire racks.