It might be because you didnt reprogram your truck cause of a smaller tire. When i went from 245/40/18 to 215/40/17 it seemed like i was going way slower. Then i bought Jet's SCU2000 programmer and it went back to normal. Also, the Hypertech power programmer will change tire size and gear ratio.
Now if you want great launch, try getting 4.11's. They kick a**. I have 4.56's now, talk about holy shiznit. Take offs are great, but rpms get a little high around 100. I still have my top speed limiter on so i dont know if i can go any faster. To do a gear swap it cost me about $525. It takes one whole day, if you want email me, and i can get you the address for the place i went to, or just buy the orange county recycler and in the autos section they have a bunch of places that put their ads in there.
Gears = 175
Install(included gear oil)= 275
Install kit = 75