1. Heat nuts in a small nonstick skillet over medium-low heat, stirring often, 8 to 10 minutes or until lightly toasted and fragrant.
2. Arrange sweet potatoes in a steamer basket or stainless-steel colander in a 4- to 6-qt. Dutch oven over boiling water. Cover and steam 16 to 17 minutes or until potatoes are fork tender. Plunge potatoes into ice water to stop the cooking process; drain.
3. Place apples in large bowl. Sprinkle with lime juice and no-calorie sweetener; toss gently to coat. Stir in sweet potatoes, raisins, salt, and 3 Tbsp. walnuts.
4. Stir together yogurt and mayonnaise. Add yogurt mixture to sweet potato mixture, tossing gently to coat. Sprinkle with remaining 1 Tbsp. walnuts. Cover and chill 2 to 12 hours.
Note: For testing purposes only, we used Splenda No Calorie Sweetener.