To answer your question, I believe those caps are just pressed onto the hub. In reality, I dont know if it is necessary to stick your fingers in there to remove the old greese? If im not mistaken, the greese is pushed from the back out to the front. The old greese should just ooze out and you should be able clean it up with out sticking your fingers in the hole. It would be a good idea to check with the manufacture of that axle to find out the apropriate method of greesing it.
'97 Jayco Eagle 314BHS
'03 Tahoe LT 5.3L