Silverlight Expander Adjust Grid

I am trying to have a tab control that fills the entire width of the browser and an expander on teh right hand side. When expanded it would push the tab control over so it does not just go on top of the tab control.  I have reviewed many samples but cant see what I am missing.

<navigation:Page x:Class="Dashboard.Page1" 
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
                 xmlns:ig="http://schemas.infragistics.com/xaml"
                 xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"
           xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
           xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                   xmlns:controlsToolkit="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
                     xmlns:controls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls" 
                     xmlns:igGrid="http://schemas.infragistics.com/xaml" 
           mc:Ignorable="d"
           xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation"
           d:DesignWidth="640" d:DesignHeight="480"
           Title="MO Page">

    <Grid x:Name="LayoutRoot">
        <controlsToolkit:BusyIndicator x:Name="cmd_isbusy" Content="" Margin="0,36,0,-3" />
        <Button x:Name="cmd_parameters" HorizontalAlignment="Left" Margin="8,8,0,0" VerticalAlignment="Top" Width="130" Content="Change Parameters" Click="cmd_parameters_Click"/>
        <Button x:Name="cmd_dashboard" Margin="0,8,8,0" VerticalAlignment="Top" Content="Back To Dashboard Selection" HorizontalAlignment="Right" Width="180" Click="cmd_dashboard_Click"/>
        <Grid ShowGridLines="True" x:Name="TemplateArea_" Background="Transparent">
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid Grid.Row="0">
                <Grid x:Name="ContentArea_" >
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="Auto" />
                    </Grid.ColumnDefinitions>
                    <Grid x:Name="DataGridArea_" Margin="0" Background="Transparent" Grid.Column="0" >
                        <controls:TabControl Name="MOTabControl" Margin="0,37,0,0">
                            <controls:TabItem Header="MO HDR" Name="MoHdrTab">
                                <Grid>
                                    
                                </Grid>
                            </controls:TabItem>
                            <controls:TabItem Header="MO Planning Data" Name="MoHdrPlanning">
                            </controls:TabItem>
                            <controls:TabItem Header="MO Allocations" Name="MoHdrAllocations">
                                                      </controls:TabItem>
                            <controls:TabItem Header="MO Serial / Lot" Name="MoHdrSerialLot">
                            </controls:TabItem>
                        </controls:TabControl>
                    </Grid>
                    <controlsToolkit:Expander HorizontalContentAlignment="Left" ExpandDirection="Left" IsExpanded="True" Margin="452,36,0,0" Width="185" HorizontalAlignment="Right" Name="moexpander" Grid.Column="1">
                        <controlsToolkit:Expander.Header>
                            <TextBlock Margin="-158,0,-153,0" VerticalAlignment="Top" Text="Available MO's" TextWrapping="Wrap" Foreground="#FFE1B3B3" RenderTransformOrigin="0.5,0.5"
Grid.ColumnSpan="1" HorizontalAlignment="Center" Width="189" Height="20" d:LayoutOverrides="Height" Grid.Column="0">
<TextBlock.RenderTransform>
<TransformGroup>
<ScaleTransform/>
<SkewTransform/>
<RotateTransform Angle="-90"/>
<TranslateTransform/>
</TransformGroup>
</TextBlock.RenderTransform>
                            </TextBlock>

                        </controlsToolkit:Expander.Header>
                        <controlsToolkit:Expander.Content>
                            <ListBox x:Name="lstmonumbers" ItemsSource="{Binding molookup}"  HorizontalContentAlignment="Left" Width="159" HorizontalAlignment="Right">
                                <ListBox.ItemTemplate>
                                    <DataTemplate>
                                        <sdk:Label Content="{Binding MO_ID}" />
                                    </DataTemplate>
                                </ListBox.ItemTemplate>
                            </ListBox>
                        </controlsToolkit:Expander.Content>
                    </controlsToolkit:Expander>
                </Grid>
            </Grid>
        </Grid>
    </Grid>
</navigation:Page>

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