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So you always lock out the OD when you travel this route, both in 2wd and 4wd? If so that shouldn’t make any difference between the two data sets. If you only lock it out in 4wd that is probably part of the MPG difference right there.
Assuming you always lock it out of overdrive in 2wd or 4wd, since you don’t have manual locking hubs, your front axles and CV joints will be spinning all the time anyway, so those wouldn’t make a difference between the two data sets.
The only difference should be the front differential would be engaged by the actuator and your transfer case should shift. This would result in your front drive shaft spinning, front differential being engaged and spinning, transfer case chain engaged and spinning, and meaningful power being transmitted to the road by these components.
From my experience with 4x4 trucks, with everything in working order I would not expect to see a 2 MPG or approximately 20% drop from solely this difference. Either the other factors (weather, road conditions) are causing some or all of the drop, or something isn’t functioning correctly.
Have you checked or changed the fluid in your differential and transfer case recently? In super cold weather my truck feels very “sluggish” when I am coasting until the transmission, transfer case, and both differentials get warmed up a bit.
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