African diplomats at the United Nations were working behind the scenes yesterday to try to avoid a rejection by the South West Africa People's Organization (SWAPO) of the Western plan for the independence of Namibia (Southwest Africa).

Sam Nujoma, president of SWAPO, who conferred in Washington earlier in the day with Secretary of State Cyrus Vance, put off a scheduled afternoon speech to the special session of the U.N. General Assembly on Namibia as the diplomatic meneuvering went on.

After meeting with Vance and other officials including Nigerian Foreign Minister Joseph Garba acting as something of an intermediary, Nujoma told reporters he had asked for amendments to the Western proposals. Nujoma said he would state SWAPO's position on them when he returnend to New York.

State Department spokesman Hodding Carter said, "I have no reason to believe that the [Western] outline would in any way be changed. We have presented what we believe to be a fair proposal."

Numerous African nations at the U.N. are urging SWAPO in private either to accept the western plan or propose modifications, rather than reject it , to keep negotiations open.

South African Prime Minister John Vorster announced in Cape Town on Tuesday that his government accepted the plan proposed by the five Western members of the U.N. Security Council - the United States, Britain, France, West Germany and Canada.

Under the plan, 58 years of South African rule would end with introduction of a U.N. peacekeeping force, elections by the end of the year and a transition to black majority rule. The plan calls for an end to the guerrilla war conduted by SWAPO and the phased withdrawal of 20,000 South African troops by year's end.

A key issue still in dispute is the dispostion of the strategic port of Walvis Bay. Spokesman Carter noted yesterday that the plan would leave this issue to negotiations "between the new independent nation of Namibia and South Africa" SWAPO is also disputing the timetable for withdrawal of South African troops.