Edit for clarification: Because I live in Arizona and spend all of time exploring in the western US, jagged rocks are a constant on the trails. And with how heavy these 80s are when equipped, I want to have tires that will be able to find a good balance for extra sidewall & width to help distribute the load of the rig a little more while also providing a larger footprint when aired down for the sand, river washes, and the occasional snow I like to get into. But I also have a lot of highway miles getting to where I want to explore which is why I don't want to exceed 35s.
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The only reason one "needs" bigger tires is if you are not getting where you want to go on your current tires. The set of of the OP's rig is indeed ideal for his stated use and budgetary constraints. Going 315s will cause fuel mileage to decrease and uphill speed to suffer on the freeway, while not providing any appreciable difference to the capability he now has.
Now, if he was constantly bashing his diffs and skids on rocks, then the extra inch of clearance could make a significant difference. To the OP--you seem like a reasonable person and have your priorities straight--If you WANT 315s, see about getting a set of OEM wheels from the Mud classifieds, and then look for a set of decent tires on CL. Total cost: well under 1k. Buy new tires in the desired size after having A) a job, and B) running both sizes and making an informed decision.
Great looking truck BTW!