If you like the truck you have (other than the smell!), I'd recommend
another one. I believe GM updated their trucks in 2007 with the 6-speed
transmission that should net you better mileage and help with towing--note,
they still made the GMT-800 like what you have in HD along with the GMT-900
that had the 6-speed auto). I would not recommend the diesel unless you tow
a lot. Personally, I love a diesel for the fun factor (modify it), but they
are really expensive to buy and don't save much on fuel unless you tow a
lot, in my experience (we have an F350 dually diesel and our 2007 Silverado
gas engine gets better mileage except for when towing). Looks like there
are good number available in your area via autotrader: 40 quad cab
Chevy/GMCs within 100 miles.
I like the autotrac (automatic shifting into 4x4) on the = ton GM trucks,
but 10,000 lbs towing is pushing it for them, IMO.