Grease and flour two 9- by 5-inch bread pans, preferably nonstick.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes).
Set the temperature to 350F and preheat, lid closed, for 10 to 15 minutes.
Spread the walnuts on a pie plate and toast for 10 minutes, stirring once. Let cool, then coarsely chop. Set aside.
Trim the ends off the zucchini, then coarsely grate into a colander set over the sink on a box grater (or use the shredding disk on a food processor). You'll need 2 cups.
Sprinkle with the salt and let drain for 30 minutes. Press on the zucchini with paper towels to expel excess water.
Sift the flour, baking powder, cinnamon, and cloves in a mixing bowl or on a large sheet of parchment or wax paper.
Combine the eggs, sugar, oil, yogurt, and vanilla in a large mixing bowl and mix on medium speed. (You can mix the batter by hand, if desired.) Add half the dry ingredients and mix on low speed; add the remaining dry ingredients and mix until just combined.
Stir in the walnuts and zucchini by hand.
Divide the batter between the prepared baking pans.
Arrange the pans directly on the grill grate and bake for 50 minutes, or until a bamboo skewer inserted in the center of the breads comes out clean.
Transfer to a wire rack and let cool for 10 minutes, then remove the breads from the pans. For best results, let the breads cool completely before slicing.