@smoothatonalsnd said:

First off, very coincidental that your name is Sean O'Donnell, because @MikeHamad and I have a mutual friend who is a music theorist with the same name, but spelled Shaugn...

The Disease jam often goes to A mixolydian, yes. And you're totally right, this makes the transition to G major much smoother. But consider two things:
1. In rock music, players seldom solo in a purely major scale pitch space with the leading tone (in A major, G#). For various reasons that are too complex and long to get into, the lowered seventh scale degree (G in the case of Disease) is pretty much the norm in rock music soloing. There are of course exceptions to this.
2. The Disease jam is actually more of a pentatonic/blues/rock jam than a mixolydian jam as it stands normally. A typical type 1 Disease will jam in this pentatonic space (scale degrees 1-2-3-5-6 in A major, or A-B-C/C#-E-F#) which conveniently avoids the seventh scale degree altogether. Type 1 Disease jams often feature a movement back and forth between A major and D major, or I-IV. Listen to almost any version from the mid 90s for this, or also the 7/25/15 set 1 version from this summer (straight fiya!). However, when the band opens set 2 with Disease and decides to make it a type II jam, one of the ways that the band signals the desire to move the sound "outward" beyond the norm is to use modes other than pentatonic/blues scales. In this case, Mixolydian.

In summing up: "Disease" jams often begin with soloing in A major pentatonic, but as Trey decides to open up the form and move the jam into further explorative space, he'll start to add in that flat scale degree 7, G, which moves the melodic mode into mixolydian. This in turn will often be a cue for Mike to move to G, which achieves the MOD-VII that @MikeHamad discussed.[/quote]

See my other response above where I explain the importance of keeping track of modes. But, imo, DWD is always a CLASSIC mixolydian jam. This is as mixolydian as it gets. He may emphasize the A major pentatonic notes, but he is also throwing in the G notes and the D notes a lot.

So, when he goes Amajor to D major, this is not a I-> IV, but a V-> I. Since he's already in mixo, he's already playing the notes of the natural D major scale.

Another thing: When DWD's go "type II" into that funky rhythm playing space (this happens almost every time around 7-10 minutes in). Trey and the band usually shift from A mixolydian to A dorian. This is a true modulation. But it's not A major to A natural minor - it's A mixo to A dorian.