I can't help you identify the aunt, but I can tell you what the Jenks-Howland connection is (or at least what A Jenks-Howland connection is -- there could well be more than one).
John Howland & Elizabeth Tilley (Mayflower passengers) had a daughter Hannah Howland who married Jonathan Bosworth. Their daughter Hannah Bosworth married Nathaniel Jenks. According to my records, both Hannah Bosworth and Nathanial Jenks died in 1723, long before that letter was written.
Perhaps the aunt in question was a descendant of Hannah Bosworth and Nathanial Jenks -- but I couldn't even begin to guess which child of theirs might be the ancestor of that aunt.
It could of course be the case that it would be easier to identify the aunt starting first from this Zebulon and his wife.