Rush calls his show "show prep" for other talk shows, but based on his flow of topics, it sure sounds to me as though FR is his show prep.
Rush has called out FR in the past (especially on some breaking news items).
He doesn't always tell how he learns of a post or thread on DU. He may be emailed these or see an FR thread (like DU Funnies). Wall Street Journal has even hit print with some offensive DU threads still online.