[AcampoDave]

I appreciate the optimism on that Weq. Also thanks for the reply Al. The mufflers are the 16" case dynomax so on my shortbed, that about as far back as they go. The tailpipes turn up right after the exit and headup and over the axle. Then run parallel to the rear gas tank to a pair of turndowns just barely below the bumper. My desire was to build a quiet performance exhaust and for the most part I succeeded. Once the cruise RPM's surpass around 2300 the drone is gone. Conversely, rolling along below 2000 is pretty dang quiet too. The drone is not horrible but its right in the 55 to 65 MPH range and it gets old after an hour or so.
From what I understand sound flowing thru a tube has a specific range or frequency in which it will begin to vibrate causing drone. A guitar playing friend of mine compared it to a tuning fork, and to me that seems pretty accurate. Another example that comes to mind is rolling down the rear door windows of my Silverado, but not the front when at speed causing a whupping sound.
My driveshaft goes to the shop tomorrow to get balanced so after that I can get busy to see if I can slam the sound waves against themselves and bust up the drone. I read that modern cars use a similar device on their intake tracks and some also use a chamber on the intake and/or exhaust called a Hemholtz resonator designed to direct sound waves into themselves. Further reading revealed the 1/4 wave and the Hemholtz resonators to have been around for a long time and to have been successfully used in a variety of applications, automotive and otherwise.