To make dressing, combine sour cream, mayonnaise, lime juice, green onions, jalapenos, chili powder and dry mustard in a medium bowl. Season to taste with salt. Cover and refrigerate (can be made 2 days ahead of time).
Cut hearts of palm, tomatoes, and avocados into 1-inch pieces. Combine in medium bowl. Mix vegetables with just enough dressing to coat lightly. Line large platter with lettuce leaves. Mound vegetables in center. Surround with shrimp and crab. Serve remaining dressing on the side.