Watch the Southern Harmonies - 1 online guitar lesson by Will Sophie from 50 Southern Rock Licks You MUST Know
This harmonized guitar part could easily be a song intro. It features the following chords: D - C - G - A, D - C - G - D. These kind of chord progressions can be heard a lot in 1970's Southern Rock. And although it's inviting to play mixolydian here, most solo's and melodies stem from the major pentatonic scales. In this case our key is D major.