What gears? If you don't know, open glove box, the RPO codes are in alphabetical order, GT4 is 3.73(s) and GU6 is 3.42(s) - the (s) is there because of the 4x2 and 4x4 differences.
Maybe get some spray seafoam with the long straw, and being a vortec engine, just pouring it in the vacuum hose off the intake will only get the rear four cylinders. If you get the spray stuff with the straw, run the truck for 5-10 mins, take off air intake from throttle body, put the straw all the way back way past throttle plate and start spraying and gradually pull it out. Let it soak for 5-10 mins then run your truck, rev it a few times then drive it kind of hard but not reckless until all the smoke clears, the smoke is from carbon build up in cylinders and what not, it made a small difference in revs and acceleration for my 96 5.7l.
I thought maybe the smoke was a little show seafoam makes your truck do to make you believe it works, so I did it again, and the smoke was WAAY less than before.
Did you gap the plugs to .060?
What size tires? that can interfere with shifting if you are running stock gears with big tires.