XAML Layout with ListBox datatemplate

I am developing a XAML based app and I have a spacing/layout issue.  I currently have my main page as a 1 column grid.  I have a ListBox though in which I need my data template to be displayed over 2 columns.  The first data element should take up 75% of the screen and the second data element should take up the rest.  When I run my app, the data from my listbox items are bunched on the left hand side of the screen.  How can I change this behavior?

    <Grid x:Name="LayoutRoot">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="2*"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="9*"></RowDefinition>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="12"/>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="12"/>
        </Grid.ColumnDefinitions>
        <!--My Section-->
        <Grid Grid.Row="3" Grid.Column="1">
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <TextBlock Grid.Row="0" Grid.Column="0" Text="Selected Location" FontWeight="Bold" Style="{StaticResource grayTextBox}"/>
            <Line Grid.Row="1" Grid.Column="0" Stroke="White" StrokeThickness="1" />
            <ListBox Grid.Row="2" ItemsSource="{Binding Path=SelectedLocations, Mode=TwoWay}" HorizontalContentAlignment="Stretch" Visibility="{Binding Path=IsSelectedLocationAvailable, Converter={StaticResource visibilityConverter}}">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <Grid ShowGridLines="True">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="*" />
                                <RowDefinition Height="*" />
                                <RowDefinition Height="*" />
                            </Grid.RowDefinitions>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*" />
                                <ColumnDefinition Width="Auto" />
                            </Grid.ColumnDefinitions>
                            <TextBlock Grid.Row="0" Grid.Column="0" Foreground="#ffb107" Text="{Binding Path=Name}" />
                            <TextBlock Grid.Row="1" Grid.Column="0" Foreground="#ffb107" Text="{Binding Path=Address}" />
                            <TextBlock Grid.Row="2" Grid.Column="0" Foreground="#ffb107" Text="{Binding Path=CityStateZip}" />
                            <StackPanel Grid.Row="0" Grid.Column="1" Grid.RowSpan="3" Orientation="Horizontal" HorizontalAlignment="Right">
                                <Rectangle Fill="Orange" />
                                 <Rectangle Fill="Blue" />
                            </StackPanel>
                        </Grid>