To venture off the main roads in this area (that is anywhere except the C14, C28, D1982 and D198), you need a permit from the NWR. These cost N$90 per site per night, plus park fees. They are easily obtained from the NWR in Windhoek; the park office at Sesriem; Swakopmund's Namib i information centre or the Hans Kries Service Station on Sam Nujoma Avenue; or in Walvis Bay from Suidwes Diensstasie (13th Road and 10th Street/Nangolo Mbumba Drive) or CWB Service Station at 124 Sam Nujoma Avenue. These permits allow you to venture on to the park's smaller roads and to camp in any of the area's sites. Most of the roads are navigable by 2WD, while only a few around Gemsbokwater and Groot Tinkas are classed as 4WD.
Even these are probably negotiable with a high-clearance 2WD and a skilled driver, though you'd be waiting a very long time indeed for anyone to pass by if you became stuck.