I don't know the part name but it's the one that's cracked. Should I be concerned to replace it asap and what's the part called. I know the cv boots leak but hadn't noticed the cracked (housing or cover)?
[IMG]https://cimg1.ibsrv.net/gimg/www.ford-trucks.com-vbulletin/2000x1504/80-20160529_114426_82a4

0fb9785a308247aadbc2aef1474da6eeb88.jpg[/IMG]

 

Ps That fluid is PB blaster I sprayed.

 

that's the axle vaccum hub , its what locks the axle in when its in 4wd.