Hi Bill,
Welcome to TBN. There is a forum specifically for Yanmars. It is a bit further down on the column, but there are guys there that know everythig about Yanmars and any difficulties they may have. If you post there I am confident that you will get a correct answer.
I have heard that there is often a problem with the diff lock if it is engaged while moving which shears something, but I do not remember the specifics. Are you sure that the pedal is traveling to its fullest?
Mike