Im dealing with the same problem, although my vehicle is mostly stock. i to take long road trips and i am planning of trying to make my vehicle more aero dynamic like grill block, belly pan, ect..
im assuming you use your Xterra off road like i use my land rover. One of the first mods im looking into to improve all around gas mileage is an electric fan swap, do Xterra's come stock with those?
also good for water crossing, i had one on my 99 jeep and got 2mpg highway from the swap.
Also im going to be making a grill block out of plexiglass that will be easily detachable. Not sure how much i will gain though. I know all terrain tires arent the best thing for mpg, but i have noticed that making sure they are inflated to the highest psi while still wearing evenly helps ALOT! i just keep a small pump to air them up after off road use.
glad to see another suv off road enthusiast on here, i though i was going to seem crazy trying to get more mpg!