Two American Peace Corps volunteers were killed when their car crashed down a mountainside in South Africa, U.S. officials said today. Juanita Quiton, 39, of Portland, Ore.; and Brenda Sue Crawford, 27, of Frostburg, Md., were killed Friday afternoon while on their way to a vacation in Zimbabwe, said Peace Corps officials who held a memorial service for them today. They had worked in Swaziland, a small mountain kingdom bordered on three sides by South Africa and on the fourth by Mozambique. Zimbabwe is to the north. The service was in Mbabane, the Swaziland capital. Two American men were traveling in the car with the women, but embassy officials did not have their names and did not know who was driving. The men were not injured. Officials said the car failed to complete a turn on a moutain road and plunged 450 feet down the hillside near the South African town of Belfast. Miss Crawford had worked since June 1987 with the Swaziland Department of Land Utilization and Natural Resources as an urban planner. Miss Quiton arrived in October 1987 and promoted home-based industry at the Sithobelo Rural Education Center. An embassy official said the bodies were expected to be flown home Tuesday.