How do I Add a Tooltip To a DataGridTextColumn

Yes, you can add tooltip text to DataGridTextColumn – just stylize it

<DataGridTextColumn Header="ScreenName" Binding="{Binding ScreenName}" >
    <DataGridTextColumn.CellStyle>
        <Style TargetType="DataGridCell">
            <Setter Property="ToolTip" Value="{Binding Name}" />
        </Style>
    </DataGridTextColumn.CellStyle>
</DataGridTextColumn>