Chevrolet Forum and Chevy Talk are good forums for discussing the Chevy 2500HD. Both websites have subforums that are dedicated to the discussion of Chevrolet trucks.
Both forums are fairly active, with hundreds of online users at any given time. While registration isn't required to view the threads in either forum, users have to register to post. Click Register on the top right at Chevrolet Forum and above the forum categories at Chevy Talk to sign up for free. Chevy Talk also offers extra features for paid users.
Both forums are moderated. As such, users must read the forum rules before posting.