Mine came out, too, a long time ago. I never bothered to put it back. Should be OK without it (I hope). No problems yet, anyway.
Although it does make it a little more difficult to slide the axle in. Mine came out of the long side. It acts as a fulcrum to keep pressure off the oil seal when you push the axle into the third member.
The other one prob. got ground up. It happend to me about a year ago, I think I knocked it out of the housing when I installed my longfields. I was in JV at the time, so I went wheeling right after they were installed. When I got home and serviced the truck, I found LITTLE slivers of that part stuck to my drain plug!
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