I have a 2002 7.3L Diesel Excursion with 91K miles. It is used primarily as an off-road recreation vehicle, desert romper and camper rig. It has a 4” lift, 35” tires on stock wheels and 4:10 dif gearing with Airlockers. The motor has a Superchip tuner and K&S air filter. Recently, I have gotten a very loud rumble from the rear of the car while accelerating at freeway speeds. Could this be the Muffler going bad? The only other think I can think of is the rear end gearing is not done right, binding or something.