There are tons of private companies that can do airport t-fers from Sandton to JNB.

One I've used for years (literally) is Dreamscapes, ZAR 340 one way. Inquire at [email protected]. They use regular cars, however they're professional, clean, on time or early (important to me), aren't overly chatty, very safe, and adjust the temp. to however you like it.

Will