Dear SOS,
I was last in the RSA in April. The cool thing is SA Air now has on-line booking. I was able to go to their web site and book (from the USA) my flight tickets for in-country travel - in my case Cape Town. It is ticketless system, and when I arrived in the RSA, I was able to go from the International Arrivals part of OR Tambo (JNB) airport, to the Domestic Departures area, where I checked into my connecting flight. Just make sure that you have with you the same VISA card that you booked your flight with.
As Chrisg wrote, you can indeed take the Great North Road. This appears on maps as the N1 highway. This is a toll road, and the toll fees will probably run you around R80.00 (about $11-12.00USD). You can hire a car from JNB. All major car hire companies are there, and you can book on-line from the USA. I have had used Hertz, Avis, and Enterprise, while there with good results. When I was last at the Polokwane Airport, Hertz was there, so there's even a chance you could pick up a rental car at JNB and drop it off at Polokwane, if the airport is your destination.
Cheers!