I wonder if they checked the connection of the exhaust pressure sensor. This seems to be the leak that most of us have been having. Crawl under your truck from the passenger side just behind the front right tire and follow the exhaust tube straight up. At the very top you will see the exhaust pressure sensor tube and the nut that connects to the exhaust down pipe. If you lean up on a shoulder you might be able to get your finger on it. Look for soot around the connection. Many have reported the nut completely loose. Mine was... it wasn't even connected... I could spin the nut with my finger. If that is where its leaking you can tighten it yourself. Its tough to reach. You might have to pull off the tire and remove the splash cover to get a little more room to reach it. Or... take it back to the dealer and let them futz with it. Somewhere here or on one of the other forums is a pic of the connection.