I've been on a few forums (in fact i'm a member of 4 of them)... I just wanted to share a couple that I thought were mentionable...
The Diesel Place... A very large forum site mostly dealing with Duramax engines and GM trannys - www.dieselplace.com
Diesel Bombers, a mods forum site with Duramax, Cummins, and Powerstroke categories - www.dieselbombers.com
I have no involvement with these sites other than being a member... btw, I am "Dmaxtothemax" on both of them, if you join recommend me and friend request me...