Peter,

Your instinct about the relative minor is correct. Regardless of the details from a diatonic analysis, the note "evokes", if I may use that word, the relative minor of D (Bm) and its dominant (F#m). This triad is one that Dylan was terribly fond of because it is kind of poetically mysterious sounding or something. Just try running from G to Am to Am-open-on-the-second-fret (Bm) and you'll find the mystery to scores of Dylan tunes.

Regards,

A