Mario Tarradell

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There's a track on the new Sugarland CD, Love on the Inside, titled "Steve Earle." It's a hypertwang number, done for laughs, that pokes hillbilly fun at the acclaimed singer-songwriter's penchant for penning tunes about the women in his life. He's been married seven times, and the character in the song, hilariously embodied by Jennifer Nettles, wants to be his new Mrs.

"We were on the bus one night after the show and we were listening to his latest record," says Ms. Nettles by phone from Peru, Ill., during a tour stop with Sugarland partner Kristian Bush. "Kristian is a big fan, and I got into him from Kristian. We were talking about him and about how he writes all these songs about his loves and his breakups. He's this bleeding-heart romantic who falls in love with these women. That, in and of itself, is a song. We thought we would put it as a hidden track but then everybody was, 'No, this needs to be a part of the sequence of the album.' "

"Steve Earle," as well as other Inside cuts such as "Joey," "Genevieve" and the heartbreakingly beautiful "The Very Last Country Song" detail the remarkable artistic maturity of this evolving musical entity. While Sugarland's debut disc, 2004's Twice the Speed of Life, was a catchy if underwhelming effort, 2006's Enjoy the Ride showcased creative upgrades, particularly Ms. Nettles' emotionally wrenching performance on the raw ballad, "Stay." Love on the Inside is the clincher, a fully defined record encompassing modern and traditional country with splashes of pop and rock.

Ms. Nettles, the mouthpiece of the pair, calls Twice the Speed of Life "an experiment." Back then they were a trio with former member Kristen Hall. Enjoy the Ride, Ms. Nettles admits, was made "under such duress," with all the pressures of quickly following a highly successful introductory CD. For Love on the Inside, their sales clout bought them time and luxuries.

"We planned ahead in our schedule," she says. "We started writing songs for Love on the Inside after Enjoy the Ride came out."

The group recorded it in Atlanta, their home. "So we slept in our own beds. We didn't want it to be superslick. We wanted it to be pretty raw. We also recorded this album live. We went in with the musicians. What you are hearing is the recording of the songs in the moment with my singing live. We didn't get to do that all of the time with Enjoy the Ride because we were working, and it was exhausting."

Inside already sports No. 1 single "All I Want to Do" and sold 314,000 copies during its first week in stores, according to Nielsen SoundScan. "Stay," buoyed by a stark, stunning stage stint at November's Country Music Association Awards in Nashville, pushed the duo into the realm of pop culture. It cleared the path for Inside.

"What it did for us, for me as a singer, for us as co-writers and each of us as individuals ... when you see the comedian suddenly do the dramatic role ... it totally grabs you and shakes you to your foundation," Ms. Nettles says. "From a writing standpoint, producing standpoint and singing standpoint, that song is different enough for people to say, 'Wow, these guys are more serious.' It opened up a whole new world for us as songwriters and artists."

Fame does bring more responsibilities and complications. Former member Ms. Hall filed a $1.5 million lawsuit late last month in Atlanta against Ms. Nettles and Mr. Bush. Ms. Hall claims she has an agreement that says she was to get a cut of the profits even after she exited in 2005 for a solo career. That, no doubt, must weigh on the future of Sugarland, which Ms. Nettles says the current duo is already pondering.

"We definitely try to think forward. We can do a good record, but we can back it up with an excellent show. We are enjoying the shows now because now we have the resources to put on a production. We know how to do a show in clubs. But we never had the resources to do a show with lighting and video," she says. "We want to expand our fan base. We want to do stadiums, to continue to evolve as writers and grow our craft. We are definitely exploring who we are as artists and are continuing to define our sound."