Hi, Renfield. Which particular JTR letters are you referring to? There are dozens still in existence.
The Lusk letter was subjected years ago to an examination by Thomas Mann of the World Association of Document Examiners who came to the conclusion that the writer was semi-literate and not disguising his handwriting.
Sugden said in his book on Jack that a graphologist Sue Iremonger, of the same Association as Mr Mann was engaged in a study of the main Ripper letters but I don't know whether those results were ever published.
If you are really interested in this aspect of the Ripper mystery, try and get hold of 'Jack the Ripper: Letters from Hell' by Evans and Skinner. It examines and reproduces all the letters signed 'Jack' and the Lusk letter. Also there are letters written by various suspects (originals) such as Bury, Druitt and Tumblety. I really recommend the book and it's available on Kindle.
The trouble is of course, who knows whether Jack ever bothered communicating with the authorities over any of his 'work'. If he was a semi-literate local man would he have bothered?