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Looking at the picture closer, center section does look like an HO52 as Nanook said. If truck is original 1 ton the housing probably an HO72, but really does not matter as all interchangeable. Only difference is bearings in the center section, which are mostly available , but as mentioned, not cheap. Heard rumors of 3.90 ratio for years, but never actually seen one, so not going to say for sure ether way.