There's a wire that goes there, it works like a retaining ring...most of the little plastic ridges in the hub cap (Plastic thing that caps your hub....) get bent/knocked off, so they don't have a good hold on the retainer....hence its loss.
Look at the inside of the hub cap, see if there's anything left to hold the retainer...you can bend a piece of spring wire to replace it if there's enough mounting ridge left.
If you just want to buy the spring wire, I doubt they are available w/o a cap...so, you'll most likely just replace them.
PS - I was looking at the blow up of your hub above...and based upon the available magnification, it appears that 3 of your molecules are out of place. Might want to have that looked at.
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- TJ
2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....
Friends don't let friends drive stock.
http://www.gifsoup.com/view/501230/tj-tackling-crawlers-ridge-o.gif