Are you implementing propertychanged on WellboreDiameter_Unit?
Also can you try using SelectedItem instead?
Edit - Sorry looking at your style again, I think there is something wrong with the style for ContentPresenter. I will have a look again and see if I can help
Edit - Your ContentPresenter is wrong, Change it to below
<ContentPresenter Name="ContentSite" IsHitTestVisible="False"
Margin="3,3,23,3" VerticalAlignment="Center" HorizontalAlignment="Left"
Content="{TemplateBinding SelectionBoxItem}"/>
Make sure you add the Content="{TemplateBinding SelectionBoxItem}" and you should see the selectedItem