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From experience with the slip yoke in the middle of my 2 piece drive shaft, there is an oring inside of the dist cap on the female end of the drive shaft. That is what retains the grease. It slips onto the male end, you put the male in into the female end, and the dust cap threads onto the female end on the tip. I can post pics probably tomorrow if you need. But I believe the part you need is a Dust Cap.
I was replacing the carrier bearing on my drive shaft and noticed the same thing. It appears there was a plastic or rubber seal under the cap. I went to the parts store and they looked at me like I was stupid. Any ideas?I have an 87 2wd lwb 700r4