During arrival at Ft Lauderdale, the aircrew attempted to extend the gear, but at first, they were unable to get a safe 'down' indication for the right main gear. After about one hour, they were able to insure that both main gear had extended and were locked. They diverted to Miami, but by then, they were unable to extend the nose gear by using the normal gear extension system and they had only 4000 lbs of fuel remaining. The captain elected to land without manually extending the nose gear; he was concerned about the low fuel condition and believed that he might compromise the main landing gear position by placing the gear handle in the neutral position while the 'A' hydraulic system was off. An exam revealed the rod end of the landing gear lock actuator, PN 69-30537-4, had failed. A metallurgical exam revealed the rod end had failed from fatigue in the threaded area where corrosion had occurred. Also, an electrical failure was found in the landing gear logic resolution circuit that would have caused an improper indication of the right main gear.