After a prominent Manhattan synagogue made public the results of an investigation into the "sexually predatory" behavior of one of its former rabbis last spring, the Reform movement took the unprecedented step of hiring expert law firms to investigate three of its main institutions. The law firm Debevoise & Plimpton will probe the Union for Reform Judaism, which is the congregational arm of the movement. The other two are the Central Conference of American Rabbis, the Reform movement's rabbinic organization, and Hebrew Union College-Jewish Institute of Religion, its seminary.