Bad alternator or battery???

[E53blue4.8is]

Hey guys, I have a 2006 4.8is that I've had for a couple of months. Its had an intermittent issue that when driving all of a sudden, the dsc off, 4x4, and brake lights come on all at once. The radio screen would also go blank and I'd lose radio sound too. After all that, it would kick into trans failsafe mode. It's been starting to do it pretty every time I drive now. And if I shut it off while the lights are on, the battery is dead and I'll have to jump it. I'm not sure if it's the alternator that isn't charging, or there's a bad cell in the battery or something. Both were new in November and only have about 5000 miles on them. The previous owner had them installed at a midas location. Thoughts??