I remember that at Crocodile Bridge there was list of options of hiring 4x4 and or guide. Rates are quite expensive - like starting from US $500 per day.
So if you are staying at the Marriott place, you need to clarify what are options to be picked up and drop off at SKZ and do not pay day visit fee if you do not plan to to the park that day. Probably the best option, if such is available - would be to be picked up and dropped off by lodge vehicle on your arrival and departure at SKZ. You can do this as well for the days you want to do self-drives.
Not to dispute what Johan says, he has much more experience in this matter. My experience driving up and down from Crocodile Bridge to Skukuza and back. plenty of giraffes, zebras and elephants. Here is one photo taken early in the morning alongside S28/S137 road.
