South Dakota requires both UM and UIM, pays UIM as your limit less the other driver's payment, and never lets UM and UIM limits be added
South Dakota's rules are in SDCL 58-11-9 (UM), 58-11-9.4 (UIM required), 58-11-9.5 (how UIM pays) and 58-11-9.7 (no adding UM and UIM). Fault is in 20-9-2 and the deadline in 15-2-14.
Read how stacked and single-limit coverage differ in South Dakota, check what section 58-11-9.7 says, or see how South Dakota UM and UIM work.