Vibration in pedal between 1900RPM-2200RPM

[mrslurp]

I have had this problem for a month now. When you are driving, and you give gas in the 1900-2200 RPM range, it seems as if it has a little hesitation, and the gas pedal vibrates. There also seems to be a unusual noise that comes from the front of the truck (under the hood) when this happens. I am clueless on the matter. I let my Dad take it for a ride yesterday, and he had a few ideas, but I didn't think they fit. He said possibly the rotors or my exhaust. I do have a flowmaster dual set up. It ONLY does this between 1900-2200 RPMs. It does it no other time, which I figured would roll his ideas out. I am desperate at this time for a logical idea, as I had it at the Ford garage once, and they did not find the problem, and my personal mechanic does not want to look at it, because he feels it sounds too complicating and i might be wasting money on something he wont be able to find. PLEASE HELP!!!

PS:
The truck is a 2003 Ranger XLT 4.0L V6. 4x4. Flowmaster duals, 29" tires, chrome rims, k&n cold air intake, bosh plat +4 plugs, performance wires, auto.