Adding an aftrer market muffler such as the magnaflow you named will increase the NOISE both inside and outside the Jeep! Adding that muffler will NOT increase either performance or milage.. My advice is to take that sicko back to a genuine Jeep dealer. The muffler should be at least PART of the smog devices and hence under warranty.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist