The foundry we used for these is no longer making diff covers, these are no longer available. 
Super-heavy-duty cast aluminum diff covers. These offer a 1" thick lower lip that will not distort or bend like steel covers (which leads to leaks) and it weighs much less than fabricated or cast steel covers and offers much better heat distribution. These covers come with stainless mounting hardware as well as sealant. Also, these covers have separate magnetic drain plug. And separate oversized fill, and oil level plugs making maintenance a snap. These covers will fit all the Nissan m226 rear axles (titan, Xterra 6sp/offroad/pro4x, frontier nismo,6sp,pro4x). These will not fit the c200 axle. Install is a pretty easy driveway affair and should take about 30-40 minutes. Oil capacity is increased by about 1 qt, so plan accordingly.