There are more than one reason why these leak. One of the major reason being heat, if you look at the location of the exhaust Y pipe and the rear cat you see that this thing is literly being cooked. Add to that the rear exhause manifold being right above. Then the issue of having the right side axle shaft pass through the unit, any radial runout of this shaft will make those seals, that are already bathed in super heated oil, flex more than they ever should, and then once this lubricant has been baked enough times it turns into mud, add to that -20F (like we have recently seen here in Minnesota) and that mud not turns into near concrete meaning those cold seals that are being asked to flex because the CV axle shaft has excessive runout is not getting any lubrication.
I know this rambels on and on, please PM me or leave a comment if you would like more clarification.