How to access Selected Item 'Tag' from Windows Phone Listbox bound by ItemSource

When I 'hard code' the ListItems inside the ListBox I am able to access the Tag property during a SelectionChanged. But when I bind the ListBox.ItemSource I am unable to access the Tag Property.

First, I do this:

    Private Sub GetPlayers(sender As Object, e As ServiceReference1.GetPlayersCompletedEventArgs)
        lbAvailablePlayers.ItemsSource = e.Result
    End Sub

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                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <ListBoxItem Content="{Binding FullName}" Tag="{Binding PlayerID}" Padding="0,5,0,5"></ListBoxItem>
                        </DataTemplate>
                    </ListBox.ItemTemplate>

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Then, I try this:

    Private Sub lbAvailablePlayers_SelectionChanged(sender As System.Object, e As System.Windows.Controls.SelectionChangedEventArgs)

        Dim sel = DirectCast(lbAvailablePlayers, ListBox).SelectedItem
        Dim strPlayerID As String = DirectCast(sel, ListBoxItem).Tag
        NavigationService.Navigate(New Uri("/EnterScores.xaml?Opponent=" & strPlayerID, UriKind.Relative))

    End Sub

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But then I get this:

System.InvalidCastException was unhandled
  Message=InvalidCastException
  StackTrace:
       at FSPIRBWP.SubmitGame.lbAvailablePlayers_SelectionChanged(Object sender, SelectionChangedEventArgs e)
       at System.Windows.Controls.Primitives.Selector.OnSelectionChanged(SelectionChangedEventArgs e)
       at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedItems, List`1 selectedItems)
       at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
       at System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(Int32 oldIndex, Int32 newIndex)
       at System.Windows.Controls.ListBox.MakeSingleSelection(Int32 index)
       at System.Windows.Controls.ListBox.HandleItemSelection(ListBoxItem item, Boolean isMouseSelection)
       at System.Windows.Controls.ListBox.OnListBoxItemClicked(ListBoxItem item)
       at System.Windows.Controls.ListBoxItem.OnManipulationCompleted(ManipulationCompletedEventArgs e)
       at System.Windows.Controls.Control.OnManipulationCompleted(Control ctrl, EventArgs e)
       at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName)
  InnerException: 

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But I can see this:

So, is there something I am missing about binding the listbox via an ItemsSource?
I mean, if I know the SelectedItem, should I not be able to 'parse' out the 'Tag' Property [which contains the string i want]?

And as far as the screen shot I posted, I find it interesting that all of the properties associated with the SelectedItem are accessible during the SelectionChanged Event even though I did not assign all of them to the ListBoxItem. I did try to write some code to access that information, but I was unsuccessful.

I guess my questions is this:
Once I build the ListBox via an ItemsSource Bind, how do I access the Tag Property of the Selected Item during the SelectionChanged Event?

Jason