Hey guys, thanks again for the advice!
I swear I have to be one of very few, if not the only S197 owners with a 4.0L V6 that has posted about using a glasspack in the midpipe to get rid of the drone from an aftermarket axle-back muffler. I couldn't really find much about this when I tried to research it. Hopefully this will help out some people searching for this fix in future.
This really helped with the drone problem, I'm guessing a drop of about 20 decibels. It's significantly quieter inside the car and the exhaust still sounds pretty loud (and a little better in my opinion) outside the car. By quieter I mean as soon as I got in and drove it was like a different car, there was no overwhelming drone noise in the city at 1900-2100rpms and highway driving is significantly better too. The exhaust note is more consistent now with a consistent rumble at idle and under load it doesn't pop and rasp as much. Sounds slightly deeper too.
Thrush Glasspack from AutoZone: $23.10 (w/military discount)
Exhaust welding/install at local shop: $30
Time for install: roughly 15-20 minutes
TOTAL COST: $53.10 (including tax)
Video right after glasspack install
2007 Mustang 4.0L V6 with Glasspack and Pypes Street-Pro axle-back exhaust - YouTube