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I own and have driven both GMT800 and 400 series 4x4s. Here is what has worked for me in the past. Assuming your truck has the 4L60e 4 speed automatic, the best mod would be to swap it out for a stick shift, such as the NV4500. I did this in my GMT400 and picked up huge mileage gains. GMT800 trucks were available from the factory with a stick so everything should bolt up, but you would have to have your computer swapped or reprogrammed. Obviously this is not cheap.
Second, pick correct tires, with tread pattern (not size) being the most important. Stick to high quality street tires for mostly street use, I have good luck with Michelin LTX M/S2 on several 4x4 trucks. If you need an all terrain, stick to a mild all terrain, don't put aggressive mudders on a street truck they handle terribly onroad. Try to stay at stock width or less, taller is ok since you get better "rubber gearing" out of them, just remember to adjust your odometer/speedometer to compensate.
From there, make sure your truck still has the appropriate factory air dam up front since this is cheap and easy to fix. A aeroshell or some sort of bedcover would be great for aerodynamics. If you want to go hog wild swap the fleetside bed for a stepside bed and then build a custom aeroshell on that. If not a flat bedcover will help some, keep a lookout on CL for a used one cheap.
I would swap all the long term gear fluids in the truck out for a name brand synthetic, front diff, rear diff, transfer case, and transmission. You might pick up a bit of MPG, but it is just a good idea when getting a new to you vehicle to ensure you have proper lubrication.
Make sure your front radiator is not blocked or full of gunk and clean it out/replace it if it is. Your 4l60e will not like inadequate cooling and it is possible to have it constantly running hot (but not hot enough to throw a code) due to inadequate radiator airflow. Been there, done that (why I had to swap to a manual).
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