When jean Shepherd walked onstage at Princeton University's Alexander Hall recently for a two-hour monologue, his fans were ready for fun. A delegation of Levittown ladies in flowered hats grabbed their kazoos and Jews harps, the instruments Shepherd plays on his nightly New York radio show. Up in the packed balcony a clique of Princeton sophisticates in rimless glasses and engineer boots unfurled a huge black-and-white banner of Shepherd crouched demonically over his WOR microphone. A bunch of high -school athletes dropped the talk about dual exhausts and shouted the famous Shepherd password: "Excelsior, you fathead!". . .