FRWYS & GRNS

2019 1500 Sierra Denali 6.2.... Replaced muffler with Magnaflow removing flapper on axle side kept resonators. Sound is Great at start up and getting up to speed but drone is totally unacceptable at 60-70mph. Can not live with the cabin drone!!! Can you replace just the muffler to accomplish what I am looking for? If so, what Borla muffler is suggested for this truck? Why is replacing the "system" from the cat back necessary if the "system" is already there? Learning as I get up to speed!