LABOR DRIVE: The only big union drive in the county is a strike by drywallers who want into the carpenters union. In fact, with 4,000 drywallers in Southern California's huge home-building business, it's the largest organizing drive in the nation now, says the AFL-CIO. . . . Other building trades unions are waiting to see if it works, says Mike Potts of the Building and Construction Trades Council of Orange County. If it does, they may take a run at organizing their segment of the home building industry, too.