'06 Z71 Bogging Down on Take Off!

Hey Guys, It's been a while since I've actually made a post of my own having a problem. But, for the last 3-4 months I'd say my truck has had a problem with low end power on take off. It's a 2006 Chevrolet Silverado 1500 4x4 ext cab z71 5.3L v8. The only past problems I've had is back in 07 I had to replace the knock sensors at a year old, and last year ('10) around december I had to replace the mass air flow sensor. At the same time I did that I went ahead and replaced the spark plugs and wires and ran seafoam through the brake booster line (no white smoke came out). The truck has 82,000 miles now. Anyway, it only bogs down on take off, when you already have speed and stand on the gas it will shift down and take off, but from a stop it will not spin a tire or if your on a hill it doesnt even have the power to get up it. This is very aggravating when you're offroad and you cant get out of a hole because it feels like the truck just doesnt have any power to turn a wheel, it just bogs down. I've taken it to 2 different mechanics and they can't find anything wrong. It isn't throwing a check engine light or any codes. Both mechanics say fuel pressure, fuel pump, and fuel filter are ok, computer says everything is ok, and air filter/intake is all clean. I know there is definetly something wrong with the truck because used to when I hit the gas coming out of a red light I could squall those tires and now it hardly moves. I'm getting really frustrated because everyone is telling me it checks out ok. I need help or any kind of advice would be much appreciated!

Thanks Guys!:neutral: