Thank you for the info. I ended up using a different approach one of my co-workers found. I am handling the SelectionChanged event and casting e as a RadSelectionChangedEventArgs object, which does have the Added and Removed items collection.
Here is the code snippet for those interested:
sender, RoutedEventArgs e)
RadSelectionChangedEventArgs eArgs = e
// We now have access to e.AddedItems and e.RemovedItems