In a large bowl, combine 3/4 cup all-purpose flour, whole wheat flour, walnuts, brown sugar, butter, yeast and salt. In a saucepan, heat water, yogurt and butter to 120°-130°; stir into flour mixture. Add enough of the remaining all-purpose to form a soft dough.
Turn onto a floured surface and knead until smooth and elastic, about 6-8 minutes. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down. Turn onto a lightly floured surface; divide into thirds. Shape each portion into a 15-in. rope. Place the ropes on a greased baking sheet and braid; pinch ends to seal and tuck under. Cover and let rise until doubled, about 30 minutes.
Bake at 375° for 23-28 minutes or until golden brown. Remove from pan to cool on a wire rack.