It's a 96 with a 4.0 with 160k miles. I bought it in 2009 with 135k, so I don't drive it much. When I first got it I did a tune up - new plugs, wires and rotor, new motor mounts, radiator, water pump and added a trans cooler. Back then it was riding on similar 33" tall tires and got very good mpg's at 18 or so. I have added frame stiffeners, a new trans, new radiator, new a/c, and a new bumper for a winch but have not added one yet.
Recently, I have changed the sensors at the TB, installed a new exh man (even had it coated in Jet Hot), starter x2, NSS, oil pan gasket and rear main. I have not changed the fuel injectors and from last Sunday to yesterday, the a/c wasn't used and we were light on the pedal. It is not geared but I was getting the 18 mpg's year ago and it wasn't geared then.
I do have a check engine light for vacuum and working on clearing this up. The code came up as I need a P0443 for a bad cannister purge valve. Is this the reason why I could be getting bad mpg's?

 

A fault in the purge solenoid can cause poor running and increased fuel consumption according to a previous poster

You can/should also look at live Data on an OBD2 for O2 sensor, fuel trims etc

for that drastic a rise in fuel consumption, you would think its a fault, not weight or gearing