WTH, I got some time now.
OK, consider Shakedown Street. And special thanks to my good friend, Doug, who first spelled this out for me many years ago when I, too, was learning to employ modes effectively in soloing. As Doug told me at the time, "How do I know this stuff? I'm a bassist, I have to know!"
For Shakedown, Jerry's basically jamming in the key of C Major (no sharps and flats), choosing which mode to follow based off the chord progression. But he's also adding in some accidentals when he wants them to best fit certain chords and to provide tension.
The modes of C Major (no sharps or flats) are as follows. Remember modes are just the notes of the major scale with the root starting at each successive note.
The modes of C Major (no sharps or flats):
C Ionion (major scale) Notes: C-D-E-F-G-A-B
D Dorian Notes: D-E-F-G-A-B
E Phrygian Notes: E-F-G-A-B-C-D
F Lydian Notes: F-G-A-B etc etc etc for the rest of the modes
G Mixolydian [same process for the rest of the chords
A Aoelian (natural minor scale)
Now let's look at the chords in Shakedown and see if there are any accidentals and if this song is truly in the key of C:
D#7 - skipping this because Jer's just sliding between the Em7 to the Dm7 here - it's insignificant for this exercise
So, looking at the chords, we looking at all natural notes with a single flat (Bb). On a piano that is all white keys, with one black key (Bb).
Technically speaking, the key signature with only one flat (Bb) is the key of F Major.
The modes of F Major are:
F Ionian (F-G-A-Bb-C-D-E)
G Lydian (G-A-Bb-C-D-E-F)
A Phrygian (A-Bb-C-D-E-etc etc etc)
C Mixolydian (C-D-E-F-G-A-Bb)
D Aoelian (natural minor)
So, you could say that Shakedown Street is in the key of F major/D minor (one flat -Bb) and play those modes and be perfectly within the key. But they may not fit the best or have the best fretboard pattern (in fact, this is the case - keep reading).
Or, you could say the song is in the key of C major and just recognize the Bb accidental in the C9/C7 chords that adds some tension.
I agree with the position that Shakedown is in the key of C major with a Bb accidental.
I.e., since their are no Bb chords in the song but there are several root C chords, this song would probably be charted as a C major song on the sheet music - with no sharps/flats by the treble cleff on the beginning of each staff. The Bb accidental would just be noted in the measures it appeared with the C9/C7s chords.
OK, assuming now we call this a C Major song, the use of the Dorian, Phrygian, Lydian, Mixolydian modes are appropriate in the corresponding places in the song.
For the C Chorus/Jam section in particular, look at the chords that are being jammed on:
Those are the notes of C Mixolydian. And what was Jerry Garcia's favorite mode/scale? Mixolydian.
So, what I do is play and solo in the rest of the song in C Major, using the appropriate modes (D Dorian, E Phrygian, F Lydian, G Mixolydian) over their corresponding chords.
I.e., you're essentially jamming in C Major and as the root changes with the chord progression, switching to the appropriate C major mode.
BUT during the long C jam that has chords with pronounced Bbs, switch to C myxolidian.
Does this help?