The only way presently to get up the pass is by 4x4 vehicle. I have made it up the pass in a normal vehicle, however this is not recommended now since the road has become very bad. The Lesotho government is responsible for maintaining the road from the South African Border Post to the top and the 8km are VERY bumpy.
There are a number of operators that operate daily trips up the pass. Those leave at around 9am in the morning, drive you up, visit a town in Lesotho, have lunch at the Sani Top and then return down to the valley at around 3pm.
Very agile 4X4 vehicle is a must if you wish to get to the top of Sani's Pass. Hiking or walking will take at least two days to get to top from nearest lodge. You will NOT get to the top of Sani's Pass without a good solid land rover or otherwise.