We've just returned from a trip to South Africa and Mozambique and loved every second. Which parks are you considering visiting in Kwazulu Natal? We visited a few and found Umfolozi / Hluhluwe park fantastic. Beautiful scenery and great game viewing. One thing worth bearing in mind is that many of the parks in the region (as is common throughout S.A.) hunted out their game last century, turned the land over to farming and have since turned it back to game. What this basically means is that not all of the species may have been reintroduced so bear this in mind when choosing which parks to visit, depending on your interests.
I have visited CT, Garden Route, Kwazulu Natal and Mozambique and whichever you choose, you wont be disappointed. They are all equally fantastic in their own right. I have to say that I dont think that combining Kwazulu Natal with Cape Town makes the most sense. CT combined with the Garden Route, Wild Coast and Drakensburg would be a good option. A good park to visit in this area woud be Shamwari. Shamwari
Alternatively, you could drive from Jo'burg to Kwazulu Natal visit some parks, head north to Mpumalanga, visit Pilgrim's Rest, Blyde River Canyon & Kruger National Park (spectacular), and the onto Mozambique. This would also be a really good trip. You could also add a brief trip into Swaziland in there too. Snorkelling and diving in the Indian Ocean off Moz is wonderful. We went to the Bazaruto Archipelago which was truly spectacular. If you wanted to include this you could add a cheap return flight from S.A to Vilanculos on Pelican Air. Three weeks would be a comfortable time frame for this.
Hope this helps,