This is a common issue with early Duramax engines. If you wish, you can read about it on the diesel page forums - http://www.thedieselpage.com/
We have an 2005 3500 D/A and had the fuel filter clog at 18,000 miles and go into limp mode. It gave a fuel rail low pressure code on the diagnostics.
We now change the filter with each oil change and haven't had any issues.
I hope that helps.