I have a 2005 Chevy express 2500 with the HD drive train, but the GMC is the same van, There are so many things that GM improved on after 2003 that I stuck with them, one of the better things is they now have all galvanized sheet metal all brake line, fuel lines, engine and transmission lines are design not to rust, I have the hydro-boost brake system with the double piston calipers on all four wheels and the braking is fantastic, the company I used to work for had several of these ranging from 2004 to 2007 and so far not one needed brakes before 100k miles one had over 150k on it by the time it needed brakes.
But there are two major complaints I have with GM, one is they stayed with the same pin hinges on the side and back doors they had for ever, and the doors that don't get used regularly freeze up and eventually break the hinges when you try to force the door open, and the fact there welded on so its not easy to change just the hinge (I believe this was changed in 2008 or 2009 series), the second gripe is the pitman joint and the idler arm joint don't hold up and start getting loose at about 45k-60k miles, with all the great improvements they have made to the suspension on these vans they slacked a little on these two joints, I love the anti-roll system they now have, as the van stays flat around curves, it no longer feels like your driving a boat like my "94" did, the 2007 I ordered that was totaled in Dec. 2007 had all the trimmings, cruse/tilt, power windows/doors, and many other features, the one thing I will always make sure when ordering one is to ask for the locker rear axle, it makes it nice to get through the snow and mud, also power door locks to me is nice not to have to go around and manually unlock every door, also try to stay with the 8 lug as the suspension is much heavier, and I'm kind of fond of the LR4-4800 (short stroke 5300) engine as it has a good pickup and really good fuel mileage, and with the 2.73 rear axle ratio its great on the HWY, my 2007 was so comfortable that when I would go on my IAEI meetings across the country and vacation afterwords, I would take the van (without the ladder rack to save on gas) I like driving it more then driving my 2004 Grand Am GT+3 I had back then.