Hi, you could do this with a hell of a lot of work. The diff itself will not fit into the Capri axle. You would need to basically convert the Capri floorpan to match the Sierra floating axle with coil springs. Why would you want to do this as the normal setup with leaf springs is quite adequate.
errr...jus' saw a sierra 2.8i with lsd being broken and thougtht i could save a bit of dollar by taking it off his hands for less than a capri unit....nothing to do with suspension setup really, just a matter of penny pinching!!