the letter contains defamatory information provided by 2 other people who are named -- people who seem to have provided the information contained in the letter.
"Provided" or "seem to have provided"? Those are very different things.
In the abstract and without any detail whatsoever, the only possible answer is maybe.
Here are a few questions for you:
What exactly did the "defamatory information" say (please omit or change any names)?
You wrote that the letter was sent "to several people at [your] job to get [you] in trouble." What is the nature of your relationships with these "several people" (i.e., are they supervisors, subordinates, co-workers, etc.)? Did you "get in trouble" or suffer any other adverse consequences? Did any of the recipients of the letter believe the information after having received a denial/explanation from you?
If you did suffer adverse consequences, how much in actual monetary damages have you suffered?
Have you spoken to the "2 other people" to try and determine whether, in fact, they provided the "defamatory information" to the sender of the letter and to try and identify the sender? If not, why not? If so, what was the result of your discussions?