Have done everything, but fuel economy has DECREASED

Purchased this vehicle 2 years ago.  In 2010, I was
getting great fuel economy, I had no complaints.  
500-600lms (300-360 miles) on a tank.  I changed
the air filter in August, and was not getting 600-
700 kms on a tank. (360-420 miles) on highway
use.  A personal best of 740kms on a tank actually.

Roll into 2011.  My mileage was poorer, (I assumed
it was the "Winter Gas, and the additives) then I
added 4 new tires in April, and always keep to
correct pressure.   Tires are Firestone
Destinations...

My mileage was barely 500kms now, on perfect
highway conditions.  I brought it in for a full tune
up...  One of the  O2 sensor was bad.  Had this
replaced. Replaced all plugs with OEM Champion
plugs.  Replaced the fuel filter, and air filter again.  
Also had a universal joint, and rear axle seal
replaced, as it was beginning to leak.  

IT's the 4.2L SLE 4x4 model.   I started a
spreadsheet to keep track of my fuel economy.    I
burn 15L-16L per 100km on highway (at speed
limit) . This is averaging 14-155MPG on highway.  

CITY MPG  - it is now getting 9.5MPG in the city.  It
burns 24L per 100KMS.   This means I can barely
go 300kms in town for $95.00 (30 cents a KM)

Compared to what I got last August/September,
(21MPG or 10.7L per 100km) this is just horrible..

Frustrated, don't know what to do next.  Love my
truck, need to remedy - any helpful advice?  So
much appreciated....