I think if you plan to off road it with the 31s you would benefit from a small lift. 2" would be perfect for 31s and would give you plenty of flex room. Cheap way to go is a 2" spacer lift.
Another, possibly better idea would be to get a JKS 1.25" body lift with 1" motor mount lift and that would give you some more room with the 31s too. (Under $200 and pretty easy to install) Then later when you decide you want 33s (and you will) you can get a nice 2.5" suspension lift to go with it.