Steve,
I just made the switch to a Donaldson muffler, and could not be happier. My situation is a little different, I bought the truck used with a 5" turbo back Magnaflow system allready installed, and it was "ok" without a load. But when towing, it was just too loud, and droned really bad, so much you could not have a conversation in the cab. Since I allready had a 5" exhaust installed, I purchased the Donaldson M090535 from Ryder Fleet Products as mentioned in several other post on "TDR" for $79. 44 and just installed it in place of the Magnaflow. It is just the right sound level and "Tone" for my ears. I just pulled the Grapevine "Northbound" last weekend with a trailer weighing just over 7000 lbs. and it sounded great! It is not stock quiet, but it is not even close too the noise and drone of the Magnaflow... . of course 5" pipe does not help. As an extra bonus I can even hear a little bit of turbo whistle at the exhaust with the Donaldson! no such thing with the Magnaflow muffler. It is a straight through muffler "no baffle plate in the middle" and fits just fine with some custom brackets welded on. I HIGHLY recommend Donaldson mufflers to anyone who would like lower exhaust backpressures, without the noise and drone.
Dan