So sorry but none of the Islands in this group are connected by anything other than a ferry.
The easiest way from South Uist to Lewis is to drive up from Castlebay to Lochmaddy and there take the ferry to Tarbert on Lewis.
There appears to be a ferry connecting the north end of South Uist ,at Leverburgh to the south end of Lewis at Otternish,but this looks like a passenger only ferry.
I would check with the ferry company,as not all ferries in this neck of the woods carry cars.
To sum up all the islands on this side of Scotland (with the exception of the Skye Bridge) are only accessible by ferry.
If you enter Castlebay on this link,you will be able to see the routes (dotted lines) that the ferries take.Of course it won't give you the timetable,nor tell you if they are car ferries.
http://maps.google.co.uk/maps?hl=en&tab=wl