You might try looking for an American-Coleman conversion. They were the ones who made the original 4x4 conversions that GM used on the 2 and 2.5 ton trucks up into the '70s. The axle was used on many other trucks besides GM, and even on some Ag applications. They also manufacured the T case for it, in both part and full time versions. The full time version works similar to the modern 'cases, with a 2wd, 4x4 full time, 4x4 locked, neutral, and 4x4 low range positions.
If you do go with 2.5 ton Rockwells, you'll want to consider a different 'case, the 205 is far too light for that application.
