The lift and taller tires isn't the hot ticket for fuel mileage. Yes, the effective gear ratio has been reduced, as well as highway RPM, but the added rolling resistance and rotational weight is sucking up fuel. Being higher up is also increasing your air resistance.
That being said I can't see a drop down to 8-9mpg. That's super piggish even by 6.0 standards. I get like 12-14 city and 14-16 highway with all the mods in my sig and a pretty heavy right foot.
If you were overfueling you'd be getting a lot of black smoke, so I don't think that's your problem. Yes, some would likely end up in the oil, but it would burn off too, causing heavy black smoke.
One possibility that you are not going to like is a cracked injector cup or bore. This could be leading to fuel in the coolant and no black smoke. I'd check the degas bottle and see if you have any grayish junk in there. If so then pray its just a cup, as those can be replaced. If the bore's cracked it's new head time.
You will see mileage diminish with wasted front hubs, but I don't know about that much. You can test them pretty easy, just pull off the brakes and give them a spin. If you are getting grinding noises, a lot of resistance, or slop then the hubs are junk. You are not going to like what they cost either. At jobber I think I paid over $300 a piece for mine.
That clicking sound could easily be a bad bearing somewhere. What do you mean the axle spins in 2WD? How'd you discover this?
'06 F250 4x4 - 5" Flo Pro exhaust, SB Filter intake, Accufab elbow, Edge Evolution (monitoring only), SCT w/ ID custom tune, FASS 195 pump, Gillette Diesel EGR cooler delete, Sinister Diesel coolant filter, ELC coolant, updated turbo drain tube/oil feed line/STC fitting/oil cooler, ARPs w/OEM HGs, Elite coolant lines, ITP RR fuel system, RCD 175/30 injectors, Powermax, BD CCV, FICM.com FICM w/ ID tune, Elite UP, BPD water pump - 13.069 @ 101.94
'02 WRX - Outback rear disc swap, EBC green pads, DBA pillar vane rotors, TXS UP/TBE/TMIC, Perrin LW crank pulley, PPG billet steel shift forks, ACT Streetlite flywheel & clutch, K&N filter, STi Group N motor/trans mounts, TiC/Kartboy rear diff mounts/subframe lock bolts/outrigger stiffeners, Kartboy SS & all shifter bushings, custom PDX tune for Cobb AP - went 14.1 on a terrible 60ft before most of these mods; shooting for 13.50s