Options to try:

A) Different mufflers ~ Dynomax Super Turbo's tend to knock a lot of bass out the exhaust note because of the flow path inside of them and are easily going to be the quietest performance muffler -- perhaps the cheapest option as well. If you have to have a straight-through muffler then look to the Borla "turbos". Either way, tuck the absolute largest case muffler that you can squeeze under the car. Muffling capacity/volume is going to be your friend here.

B) Add resonators or another set of mufflers as far back as you can.

C) Change up a section of pipe by adding an extra bend. If you've got a lot of long and straight runs in your exhaust those can promote standing waves in the pipes and set up resonance.

D) Tie-in an exhaust venturi in the longest straight run of pipe that you have. Don't put it in the middle of the run though, do it at 1/3 or 2/3's the length of the pipe.

E) Keep the exhaust the way it is and add a sound deadener if the resonance is from an actual panel, i.e. trunk floor, vibrating.

F) If all else fails, add a J-bend helmholtz chamber.

I've been in the same pickle as you. I have a car that I tried 4 sets of mufflers on to no avail when it came to quelling a resonance. It got expensive, labour intensive, and was a royal piss-off. I think your best and cheapest option would be to either swap out to a Dynomax/Walker turbo muffler, or add some form of resonator as far back as possible in the stream.

Best of luck to you killing the resonance!