I see the V8s always get worse mileage than my 4.0 six... I got 16.5 for my wheeling trip this past weekend, but should be getting back to 20+ with just street highway driving.
I'll suggest, get rid of the AWD transfer case an d swap in a NP242 or a 231. You get a lot of driveline loss in that AWD tcase. I lose about 2 MPG when i run my 242 in the fulltime setting. I was truly surprised to see 16.5 for my trip this weekend.
Had my daughter with me, she was taking the pictures. I have had the roofrack crossbars off. I have them on for carrying some lumber... This trip with my club was to assess trail damage on a trail we adopt. We couldn't get to the lake where the trail is, still better than 8' snowpack we couldn't get past. We had a great time, with great people. And those are only 30" tires, and no lift on the ZJ, yet.