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Am I interpreting correctly that your truck will not let you go into overdrive in 4x4, or are you choosing not to go into overdrive in 4x4? Unless there is some truck specific issue not to, I would still use overdrive in 4x4 if possible, assuming you are traveling fast enough that it is necessary. My Chevy automatics had no problems in 4x4 and overdrive at the same time. My Chevy/Toyota stick shifts didn’t really have much of a say in the matter and did fine in 4x4 and Overdrive.
Are you using instant feedback (gauge or other) to determine the 2MPG loss, or is this a tank average?
If it is an instant loss, are all your tires the same brand and size? Running differently sized tires puts a lot of strain on a typical transfer case, which has no differential forcing the front and rear axles to spin the same RPM. Small differences in tire diameter will forcibly make one wheel slip after it builds up sufficient strain on the gears/belt in the transfer case.
If it is a tank average, note that it is typical to get lower FE in super cold weather, in rain, or driving through slush/snow so it may not be a 4x4 issue, just a weather issue.
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