What are your chances? "It depends". Other than wanting to be off of probation or finding it inconvenient, do you actually have a need you can articulate? Have you discussed early discharge with your probation officer? If not, why not? If so, will your probation officer support (or at least not oppose) your petition?
You should expect a letter to be treated by the court as hearsay, and should anticipate that letters will be given little to no weight.
You certainly should prepare any person who writes a letter on your behalf for the possibility that your probation officer will call them - that's much more likely than a call from a prosecutor.

