Thats pretty good mpg for a WK2, even the 4x2 models. Your Overland does not operate as a 4x4 in normal mode. 4x4 gets engaged when wheel spin is detected. So typically you are driving in near 4x2, thats 4x2 plus all the rotating front end driveline pieces and a little bit of drag in the transfer case.
In order to save mpg, you need to stop all that rotating drivel line mass that adds drag.. but there is no way to do that on the WK2... nor most other of today's AWD light trucks.
2011 Grand Cherokee Overland V8, 2009 Liberty Rocky Mt V6, 2000 Grand Cherokee Laredo I6, 1979 CJ7 I6 Quadratrac