Factory temp gauges have a reputation for being inaccurate. I dunno but the normal operating temp shouldn't be drastically affected by outside temps that aren't extreme and 2 degrees isn't extreme IMHO and especially since have you have a radiator blanket.
Like I said though, I know nothing about Duramax diesels so I can't say based on experience if it is operating normally.
If you want a more knowledgeable opinion post a question at ...
http://www.fullsizechevy.com/forum/forum.php
