I called Borla to ask what muffler they recommend based on a previous application I had. They said that they don't offer the muffler only option for the 5th gen Ram trucks like they did on my 2006 Silverado where I purchased the GM Performance Touring muffler. They recommended the full system to eliminate drone issues.
They mean they don't have a direct-fit muffler application like Flowmaster & Carven for example. But you can get a Borla universal muffler in 3" in/out and have a good muffler shop welded in like a lot of people do.
However, unless you get a full catback system, it will likely drone when in MDS (4-cylinder) mode. There is a reason why the stock muffler looks like a oil barrel, which is to control drone in MDS operation. The higher end full exhaust systems are tuned to avoid or reduce this drone in MDS. Of course, if you turn off your MDS, it won't drone with most aftermarket mufflers just welded in place.
Personally, I think the Flowmaster Outlaw muffler sounds the most like the Borla ATAK. It sounds like a 60s muscle car when you're on it, but doesn't drone when cruising or highway (in 8 cylinder mode). Whichever muffler you choose, you can get one and installed for about $275 out the door. Quite a bit cheaper than a full system.