Most my jeeping buddies own Civic's or Integras, or Metros. We've found that's a much better way to get better economy from our jeeps.
I filled my jeep about a month ago.
Mine's on 6" of lift with 33's and regeared axles. I get around 12-15 with it.
Be careful blocking off the grill, Cherokee's have a very limited cooling system, and you don't want to overheat it.