How much air pressure should I run?
No it isn't what's posted on your door, and proper air pressure will be different from front to rear. You don't want to wear out those brand new mud terrains do you? Easiest way to find out is to do a chaulk test. Now you can use just about anything, paint, chaulk, grease pencil, whatever you prefer. I typically just end up using some sidewalk chaulk. With chaulk you can't drive very far, as it wears off very quick. Using a bright color draw a good thick line across the tread of your tires. Depending on what you used to make the mark, sometimes just rolling it a few feet in the driveway is enough, other things you may have to drive it up the road a little ways, then inspect your mark. If the mark is gone in the center, you have too much air in your tires. If the mark is gone from just the edges you don't have enough air in your tires. The idea here is that if all the tread is touching the ground it will wear off your mark evenly. If you don't have the correct amount of air pressure in the tires, just certain sections of the tread may be touching, which will cause those sections to wear quicker.
This image may help some:
Also realize tires will grow some at higher speeds so you'll still need to keep an eye on the tire wear and adjust your air pressure as necessary. Just to throw out some numbers, on my 37x12.5s mounted on 10" wide beadlocks, I've found 25 PSI in the front and 20 PSI in the rear works out very well for me.