Is there a good performance muffler that's quiet?

The problem isn't the Flowmaster muffler.

The problem is where you have it exiting, out the side before the rear wheel.

It will be much quieter and have very little change in exhaust flow, if you change it to exit out the rear by the bumper, instead of where it is now.

Leave the Flowmaster, but cut the pipe before the bend that routes it out the side and have a new pipe made to go back over the axle and exit out the rear corner of the truck.