If you can really "easy" get $5k for your truck, you must be in a much better market than here ;-)

One note, if you do go larger on the tires, carefully look at the weight you could be adding, that unsprung difference can matter as well. There is a lot of variance between mfrs and styles; if you stay as skinny as possible that can help.
In BFG MTs, the diff between a 33x12.5 and 33x10.5 is 9 lbs!

In my experience, w/ these motors your are definitely better overgeared than undergeared, I've found the best power efficiency around 32-3500. Not sure how that equates to fuel efficiency though.