Honestly with an 8" lift and a 3" body lift, I bet it's just to look cool. That is too much lift to be a practical wheeler, in my opinion, it's too top heavy. I think it's too top heavy for street use even. And unless you have stronger axles under the truck, then with that lift and 37/38's, you aren't going to make it far before you break.
That being said, I'm sure you could fit 37's, but I'd question the fitment of 38's, as well as the durability of the axles with EITHER tire.
But that is just a guess that make sense on my part, I don't know for sure.
But, if you trim the fenders, I bet you could easily fit fit 38's or bigger.