I have just returned from a round trip starting in Malawi travelling through Zambia, Namibia, South Africa, Swaziland, Zimbabwe, and back to Malawi.
The road conditions were good in all the countries and although we have 4 wheel drive we didn't need to use it.
All the main roads in Malawi have been resurfaced and are excellent. If it's the raining season there are some roads in the Game Reserves that you may need 4WD.
Roads in Zambia are fairly good - some were a bit pot-holed but again they were working on their roads.
Namibia roads are excellent - even the dirt ones
South Africa & Zimbabwe both have good roads.
Hope this helps - let me know if you need any additional information.
Have a good trip.
Regards
Vera