2004 Frontier Drive Shaft Lubrication

[mikewvu]

I recently crawled under my frontier to locate the drive shaft and noticed that there are no grease fittings anywhere which I assumed meant that it was sealed and didn't require any maintenance. However, I got to thinking about it and there were some "nub" that kind of looked like plugs near where the grease fittings typically are. However, they're made of metal and would not remove easily. Does anyone out there know what these are, or if I even need to lube the drive shaft on a 2004 4wd frontier?

Thanks.