Since neither Bates nor the regimental records I have seen mention Schultz, I am not at all confident that he actually served in the 91st. Note that the National Park Services Civil War Soldiers and Sailors System (a transcription of the index to the compiled service records) also does not include a John Schultz in Company B of the 91st (accessed 12 June 2011).
He served as a private in company B (1, 2).
Perhaps he is the John B Schultz who was born on 25 September 1820 and died on 26 November 1871 and is buried at the South Bellegrove Cemetery, Bellegrove, Lebanon County, Pennsylvania [see Find a grave memorial 14759482; note that Anna Schultz is also buried in that cemetery, and her Find a grave memorial (14759487) mentions that she was the "[s]pouse of John B".
On 28 January 1875 (or 1895?), his widow, Anna Schultz, successfully applied for a pension (1, 2).
1 pension index, by regiment (John B [?] Shultz)
2 pension index, by name (John B Schultz)
not in Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233.
[I did not find his widow in Ancestry's index to the 1890 veterans' census (searched October 2005). There was an Annie Schultz in Cleveland Ohio, but the census included no more information about her]