If the Cold War heats up during the late 60s early 70s rather than the detente of Nixon, South Africa could be seen as a much more strategic partner of the West. For instance, if the Soviets intervene during the Yom Kippur War and Egypt remains firmly pro-Soviet leading to a longer closure of the Suez Canal, shipping from Asia to Europe will have to sail around the Cape. This might even lead the Brits to renew the Simonstown Agreement past 1975.
After 1975, the departure of Portugal from Angola and Mozambique removed an important buffer from both Rhodesia and South Africa. The Portuguese presence in Angola protected Southwest Africa (Namibia) and Mozambique of course acted as a buffer for Rhodesia. I imagine that the U.S. would have to actively intervene to either support the Portuguese more whole-hardheartedly or help install some pro-Western regimes in both countries immediately after 1974. Butterflying away Jimmy Carter would also be important as his presidency was not as cozy with South Africa and the Rhodesian government. Nixon's foreign policy led by Henry Kissinger was more than willing to accept the two nations as being better alternatives to possibly Communist or even Non-Aligned regimes.
There were several ways in which the United States could assist South Africa and Rhodesia, first and foremost would be to veto any resolutions in the UN which could be construed as being too harsh to either country. With regards to armaments, IOTL there were weapons smuggling deals where Rhodesia was trying to acquire F-86 aircraft from Venezuela. The US government could have simply looked the other way and feigned ignorance when these weapons arrived. Of all of the Western Countries, France was the most willing to sell armaments to South Africa (Mirage III) and perhaps the U.S. could encourage the French to maintain aircraft sales to SA.