Plastic center caps are for pussies. IJS.
I don't see why the center caps wouldn't fit in the rear. They should. You may have to remove the factory dust cap, but they will fit. If not, the factory dust cap is a pretty good center cap as well.
The problem is usually in the front because the axle sticks out too far. If I were you, I would have no problem cutting down the long end of the axle to make the center caps fit. That won't hurt anything.
Why don't they fit though? What's what we need to find out.