I think he's playing a single D root note over the F chord, that would be a slide up to the D at the 12th fret on the D string, and doing a counter rhythm to Jerry. I may be wrong. Then I thing he accents the the third we Jerry's back chording the 1 chord (the D). I could be totally wrong. It gives an augmented background colour to Jerry's chord work. That would be a 6th over the F and a 9th over the C chord in my books, but I'm no theory guy - just play by ear.
I like that Dew. I've got it on my iPhone as a practice tune. Very slow, and interesting Weir licks. He comes in intentionally late with them in the first few choruses and it works perfectly! It's a much harder tune to play slower than usual. They really slowed it down on the E72 tour. Interesting comparison to the Dew from ladies and Gentlemen (Fillmore East spring 71 shows). That's an epic Dew as well, with Jerry playing what I think is Rick Turner's Peanut guitar. PAF's on that Fillmore Dew and 57 Strat Pups on the England Dew's. They both sound amazing to my ears.