I have a 22", regardless it had a drone like yours on highway around 70/75mph.
even before I installed Long tube Headers.
I installed Vibrant Ultra Quiet Stainless resonator after the muffler on the tail pipe
It still sounds loud when you step on it. but on highway speeds, you wont believe the difference. and you will not loose power or gain any back pressure its flow threw design like the magnaflow.
they are around $90 bucks shipped on fleebay. here is how they look.
I have 2.5" you will probably need 2 or 2.25"
here is the link:
http://www.ebay.com/itm/2-25-Vibrant-Pe ... 7b&vxp=mtr