From wikipedia:
"Swing Low, Sweet Chariot" was composed by Wallis Willis, a Choctaw freedman in the old Indian Territory, sometime before 1862. He was inspired by the Red River which reminded him of the Jordan River and of the Prophet Elijah being taken to heaven by a chariot.
I have found an arrangement for trombone and piano in pdf format. Just follow the following link:
http://www.janwolters.nl/blad/swinglow-trb.pdf
It is taken from the site of Jan Wolters, a Dutch arranger, who makes free music avaliable on his website http://www.janwolters.nl/jwgratis.htm.
