The only thing inherent here is the Roush mufflers. IF you are getting rasp and you have the stock Cats, resonators and mid pipe (no off road x) and you got rasp with just those mufflers, its the muffler design. I say this because I have 3 setups on my car with the stock X pipe and cats etc and Rasp did not occur till an off road pipe was introduced and the cats and resonators were removed.
There are lots of ways to fix it this but if the mufflers are that way to begin with its gets tricky. You have to tune the exhaust and that often is trial and error. If you add more resonators which you can you will want to go greater than 12". 12" long 4" diameter resonators will cut a little rasp off the top, will reduce or eliminate the drone on the highway but will probably not remove what you want, you will want to go with a longer tube say 16 to 18" resonators or go with something like Vibrant ultra quiet resonators but you run into other issues going longer.
#1 room to weld them into stock pipes, 4" is about the comfortable diameter but only a couple spots they can be welded.
#2 length may not give you many choices where you need the sound tuned the most.
Best results are usually further away from the axle backs, but if room is tight then you will have to go just before the over axles.
Cheapest most effective way in your setup and this sucks, is to replace those mufflers they are so well known for being raspy, get a higher quality set of axle backs that work best with your setup out of the box.
Check out some other brands etc. Because your options are a pair of $100 good resonators, maybe another $150 to $200 to have them welded and you may find it did not even make a dent at the right wavelength.
Most of the posts and reviews of those Roush mufflers are the same, all rasp. Not bad sounding till you hit that register, even the V6 shelby review complained about it and from the video I can see why.