Is the deferred payment that her ex-husband paid her considered haram after discovering that he used to drink alcohol and sell it?

There is no blame upon you for taking the deferred dowry, even if the husband's money was mixed with unlawful earnings. This is because if lawful and unlawful money are mixed and indistinguishable, it is permissible to deal with it and consume from it, just as the Prophet, peace and blessings be upon him, dealt with the Jews. Furthermore, money that is unlawful in its earning is only unlawful for its earner, and it is not unlawful for someone who takes it from him through a permissible means, and the dowry you took from him is lawful for you.