That looks like an aftermarket overload setup, the factory option had the springs on top like yours. No it isn't a limited slip. A limited slip rear end refers to a clutch mechanism that is inside the differntial housing. On a Chevy it is generally called Positraction or Posi. GM did not offer it on trucks until '59 on 1/2 tons, 3/4 and 1 tons came later, although you could get them with the No-Spin (Detroit Locker) in the early '60s.
