I have run bio-diesel in my 2011 - purchased the fuel in the mid-west last year - no issues. The 2011's can run on B20. I think previous models were limited to B5. All of the D2 at the pumps should be the ultra low sulfur diesel - if it is off-road diesel it should be marked as such.
I run Standyne additive in my diesel. Was the only brand approved by GM back in 2005.
You will find allot of good duramax info at:
www.dieselplace.com/forum/index.php
www.duramaxforum.com/forum/
You will find good info about the Allison transmission on the sites too.
Would be good to read the diesel supplement owners manual.