You don't need to set ItemSouce in XAML anymore, if you are already creating the binding in code-behind. This should be enough to set the ItemSource.combo.SetBinding(ComboBox.ItemsSourceProperty, resultFeaturesBinding)
However, you need to define the ItemTemplate for your combobox. Note that this is case-sensitive and should match the field you are binding to.
<ComboBox x:Name="combo" Width="200">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding TRACT}"/>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>