We are going to Namibia in September as part of a SkyTeam RTW award in J. Well, actually we are stopping in Europe first, then flying AMS-CPT on KLM, then renting a car and driving.
I might beg to differ with an above poster on one point. In J/C, Delta's product might be superior to KLM's, even with KLM's lie flat seats. The food and hospitality will definitely be better on DL, though good luck finding award seats in that class. At any rate, it might shave many hours off of your flight.
One trick to finding availability on that route: Have thew agent check ATL-DKR and DKR-JNB separately.