Gearing for a series of possibly tough labor negotiations in light of recent television industry mergers, a local union leader representing more than 1,000 technicians, writers and producers said the local will rejoin the AFL-CIO. Local 53 of the National Assn. of Broadcast Employees & Technicians had bolted from the Los Angeles County Federation of Labor following a dispute during its 1976 strike at NBC. Local president Carrie Biggs-Adams said the union had pulled out of the federation after another union local, the International Assn. of Theatrical and Stage Employees, claimed jurisdiction over the striking workers at NBC, voted against the strike and prevented the local arm of the AFL-CIO from sanctioning the strike. Reaffiliation, she said, gives added strength in labor negotiations. Although no strike plans are imminent, she said the local faces negotiations next year with NBC.