The difference in differentials

[Tony Smith]

One solution that worked well for me a few years back on a 4wd diff housing (stamped steel, not cast housing) was to use a "donut" shaped magnet off the back of an old speaker cone. The drain plug fitted inside the hole in the magnet, and when it was time to drain the oil you take off the magnet to access the plug, thus allowing any metal particles to wash out with the oil as it drained.
I'm not sure how effective this would be with a cast housing, but even if it magnetised the whole housing and attracted the fine metal to any point of the interior wall of the housing, at least this would keep it out of the gears.