Here's a question: Are you actually verifying your true mileage by dividing the miles driven by the gas purchased between a completely fill up? Or are we talking about going by what the computer in your car says?
If it's the second one then your mileage may not have changed at all, but what did change is your driving habits. For example driving a lot on open highways at full speed has my computer up around 400 after a fill up. Then driving a lot in town more slowly with a lot of stop and go stuff will change that estimate down dramatically.
My LE is going the other way — just got 25.02 US mpg (9.4 L/100km) on trip back from cottage running at 55-65 mph, no wind but hilly.
Best I've ever got.
Truck has over 11,000 kms on it now and runs on Michelin LTX MS2. Only myself and the chocolate lab were aboard.
Major impressed.
Has your Transfer Case Fluid been changed? If so, to what? I have heard of people losing about 1-2 mpg by using 75w-90 instead of the Toyota SAE 75W LF fluid, which is like $40-50 a QT.