I know I am not going on this trip, but hear me out –
If you hit your rock sliders too hard and get a little bit of body damage you will be sad. But if you hit your differential hard and the ring gear slices through the cover you are pretty screwed. Not to mention a tranny or an oil pan failure. What do you think is more important? Body damage, or mechanical failure?
Also- I pretty much always know when I am going to hit a rock rail and can go slow, but every now and then a rock sneaks up and smashes your differential without you even knowing.
I would go for the diff covers and oil/trans before the rails since you already have rubi sliders.
My .02
My ride- 2001 power wheel, 11" plastic tires, upgraded battery, boat sides, custom bumpers, tow hooks, new paint.