Which "easy mods" have you tried with no results?
I've got basically the same truck ('03 Silverado W/T Extended cab, 4.8L with 3.73 gears) and have been considering a few eco-mods. I would be interested in knowing what other have found not to work so that I might avoid wasting my time on them.
I've read reports of 1-2mpg gains by getting the ECM tuned, and these guys seem to be the best deal:
Black Bear Performance :: Custom Tuning Solutions for 96 and newer GM Vehicles I'll probably get this done in a few months.
Mike