The main difference between ecumenism and non-denominationalism is that the former is a movement, and the latter a philosophy.
Ecumenism refers to the efforts within Christianity to reunite the Church, or at a minimum to create stronger bonds between denominations within Christianity. While the origins of ecumenism has been traced to the days of Jesus, its modern history enjoys greater clarity and consensus, particularly with reference to the Second Vatican Council. The ecumenical movement seeks total reconciliation of Christians of all denominations. In effect, it envisions the dissolution of the split in the Church marked by the Reformation, in spirit if not in deed.
Post-denominationalism, in contrast, is less a movement than a philosophy wherein all Christians are blessed and that the Body of Christ applies to all denominations of Christians. Unlike ecumenism, which seeks to reunify the Church, post-denominationalism dispenses with that broader political effort in favor of uniting Christians under an underlying fundamental belief in Jesus Christ.