The only way to get the caps off is to take a torch and heat the caps till the plastic expands and is forced out. Wear long sleeves and a face shield as the plastic can sometimes come out violently. It will snake out usually with a couple pops.
A MAP(yellow) propane torch will get the thing hot enough. Then from here on you use clips on the new u joints. Usually the axle has a recessed area for clips to go on the outside of the caps.