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IH apparently had severe problems with rear-end failures on the first 60-series tractors produced. This is discussed in some detail (pp. 144-6) in Randy Leffingwell's book, International Harvester Tractors . According to his book the most severe problems were seen in the first 3000 tractors produced, and the reengineering needed to fix the problems appears to have reached the production line by late 1959. IH also had a series of recalls to fix the problems in tractors which had already been sold. So I guess if you have one of the later tractors, or one of the earlier tractors which was repaired properly, the rear end should not have any more than normal problems. Quoting from Leffingwell's book, revisions to the 60-series rear ends included "bull gears, pinions, and brake shafts", later expanded to "differential bevel gears, revising tapered bearings, and redesigning the entire differential case". "It is known" Tractor Product Review Committee secretary A. J. Butler wrote, "that final drive failures, particularly bull gears and pinions, are caused by failure of the present ball-type inner rear axle bearing." The major problem seems to have been that IH light-heartedly grafted the old rear-end designs from the H and M tractors onto the much more powerful 60-series tractors without doing the proper engineering to make sure they were capable of handling the increased power. The main vulnerability seems to have been to sideways forces, and replacing the ball bearings with tapered bearings fixed most of the problems. However, galling ascribed to poor distribution of lubrication was also noted and apparently lead to much of the other redesign noted above. After rereading this material, I'd say the problems in the 460 and 560 models were severe enough that they got fixed. But in the case of some of the earlier tractors in this series, the fix(es) were made long after the tractor left the factory. At this late date, any 60-series tractor almost certainly had its rear-end problems (if any) addressed long ago. What might be of interest and concern to some other Farmall owners is another quote from internal IH material in Leffingwell's book: "the marginal status of the final drive components on the Farmall 300, 350, 400, and 450 series tractors was also becoming apparent after one, two, and three years of service." As far as I know, these problems were never systematically addressed by IH. At this point, I'd say that anyone owning one of these tractors should keep this in mind, and make sure that the tapered bearings eventually used in the 60-series tractors are used in any rebuild of their hundred- or 50-series tractor's rear end.
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