Just happen to have a 2008 GMC brochure here and here is a link to a site that makes trailers, where you can see some specs.
Utility Trailer Sales | Manufacturer of Utility Trailers
If your tractor + other load is approx 8,000 lbs - looks like you would be right on the line for a 5 Ton rated trailer and maybe have to go with 7 ton rating to be on the safe side. In any event assuming 8,000 load + at least 2,000 for the trailer = 10,000 lbs you would be right at the margin for a Crew Cab 1500 Series with 6.0L 367 HP. (Max rating 10,500 lbs)
The 2500 Series is rated 12,500 - 15,800 lbs depending on configuration.
The 1500 Series rating is based on a ball hitch and the 2500 Series based on 5th Wheel / Gooseneck.
Hope this helps.