When the half-hour comedy series ''What's Happening!!'' began its domestic syndicated run in 1980, it proved even more successful than its original airing on network television from 1976-79. Thus was born ''What's Happening Now!!'', in which seven years have elapsed and the cast, headed by Ernest Thomas as Raj, Fred Berry as Rerun, Haywood Nelson Jr. as Dwayne, Shirley Hemphill as Shirley and with the addition of Anne-Marie Johnson as Nadine, are now young adults in their old neighborhood. Raj is still the aspiring writer, but he becomes half owner of Rob's Place, the favorite hangout of the old gang. Lovable, assertive waitress Shirley has saved her tips and gone in with Raj as silent partner. Slick-talking Rerun is a TV commercial pitchman, "Swami Stubbs," while his bachelor-apartment roommate, Dwayne, is studying computers. Raj now has a wife, Nadine, who is a college graduate and social worker, the very profile of a modern woman. Now if these three guys could manage to grow up¦