I don't remember Citricos ever having prime rib on the menu.
We had Christmas Eve dinner there a few years ago. That night there was a mixture of their regular entrees and some specials.
I think 1900 has prime rib on their menu. But it is a buffet. Also if you're staying at the GF, prime rib used to be on the room service menu. Oh, and GF Cafe has had it on the menu too. But things do change all the time.
So they're doing prix fixe this year? Where did you see that?
edited to add that I saw you will be at WL. Citricos is excellent. But if your heart is set on prime rib, I'd check one of the mid range restaurants like GF Cafe or Kona.