Anthony, the term Rackensacker predates the Mexican War. see this from the Dictionary of American Regional English – Amazon.com “Read Inside the Book”

http://www.amazon.com/gp/reader/0674008847/ref=sib_dp_srch_pop?v=search-inside&keywords=rackensack&go.x=7&go.y=14

I believe that I recall Arkansans being called Rackensackers on some occasions in the Mexican War. (Off the top of my head) I believe that Sgt. Jonathon Buhuop's "Narrative of the Central Division" (1847) uses the term.