On a semi-floating rear axle the load is carried on the axle shaft itself. On a full floating rear end the bearings mount on a hollow spindle that the axle shaft goes through. The spindle carries the load and the axle shaft just provides the wheel driving torque. You can tell the full floating axles by the large hub that protrudes past the wheel mounting surface of the brake drum and the ring of bolts that hold the axle flange to the hub body. On the earlier trucks most 1/2 tons were semi-floating and anything bigger was full-floating. At some point in the late seventies or early eighties Chevy started using semi-floating rear axles in some of the lighter duty 3/4 ton trucks.