Iran-Contra prosecutor Lawrence E. Walsh cleared a major legal hurdle when the judge in Oliver L. North's case upheld two of the three central charges against the former White House aide. But now the independent counsel faces a more difficult obstacle posed by North's insistence that he needs access to highly classified government secrets in order to defend himself. North is accused of illegally diverting more than $12 million in U.S.-Iran arms-sale profits to Nicaragua's Contra rebels. U.S. District Judge Gerhard A. Gesell, who on Tuesday upheld conspiracy and embezzlement charges against North, Wednesday was to consider defense objections to deleting sensitive references from 350 classified government documents Walsh wants to use as evidence. North contends the references to foreign countries and U.S. intelligence agents are needed for his defense.