Ok so kind of along post but I have a 99 GNC 1500 vortex 350 4x4 3.73 gears with lunati cam , ls6 valve springs sn clips, motor has about 2k miles on it, my problem began when I put was going through a wash out on my way to a fishing spot no mud at all just a wash out an I put it in 4x4 high to come up the other side an gave I half throttle NOTHING 3/4 throttle still nothing but bogging down no tires spinning i locked it in 4x4 low an all the power was there also in 2 wheel drive it wont do a burn out just bogging, ( I only have 31's) also when I'm just regular driving after it shift into another gear if falls on its face I have to give it no less than 3/4 throttle befor it will down shift, it's also been tuned by hp tuner by a custom tune guy! Is this normal or what is going on!!!

 

kind of hard to guess without some trouble shooting. i would put a fuel pressure gauge on it and see where its at