[SS Tim]

Been doing a lot of research, unless you think the axle has been changed it is probably the factory K20 HO52 (5200lb) axle. The brakes would be 11 x 2 3/4". There was also a Dana 60 (5500lb) usage in the K20s but that is very different looking intergral carrier style axle.
A C30 rear axle, HO72 (7200lb) has a heavier housing and 13 x 2 1/2" brakes on SRW trucks. Often people cite the load bolt that controls ring gear deflection as an indicator but it isn't. Fact is the drop outs are interchangable with the difference being the housing itself, the bearings, hubs and seals are the same. At this point the easiest thing to do is check the axle code and see if it is a K20 or a C30 application. There is no code overlap I know of between the two. But the catch is the carriers are code stamped and not the housings. There is an axle code chart in the parts manual.