A little info first, my Expedition is a 99 5.4L with the A4wd, 4HI, and 4LO transfer case, unsure which model case it is.

Here is what I experienced today. While under any acceleration at highway speeds on dry pavement, I felt a vibration on my feet, coming from the front end.

I stopped, pulled the fuse for the A4wd clutch system, giving myself only rear wheel drive, all the time, and the vibration completely vanished.

I was under the impression that in the A4wd setting, the transfer case is in 2wd, but when any slippage is detected, it shifts into 4HI until no longer needed. If I am incorrect in my knowledge of the system, please let me know.

I would not have thought that the front axle would be engaging in these conditions.

Now that I have figured out where the vibration is coming from, where should I go from here? There is not a single leak on the bottom of this truck, everything is completely dry and clean, which I am surprised at, considering its 120k miles.

Being that it seems to be driveline, first check u-joints and cv shafts. Also check the tie rod ends for good measure, as a torque-induced toe on worn joints can cause what might feel like vibration as well.

My experience with AWD is that the front axle is always engaged. It is just a matter of how much. Some vehicles use a 90/10 split or a 70/30 split, and then adjust the ratio accordingly, based on rear wheel slippage.

It's been awhile, but doesn't the VSS factor into the use of AWD also?

First, the no-brainer stuff like Fellro suggested. Changing transfer case fluids would be the next step because of the clutches. The all wheel drive works by the heat generated by torque request and slippage. Imagine a container filled with hundreds of wafer thin pieces of metal. If you put one of these pieces in your hand, they'll start to curl up just from the body heat. Now, heat starts curling up all of these metal pieces "clutches". Eventually, this curling up takes out all clearance between the "container" and the clutches, locking the front axle in. So, you can see how important the fluid can be. Even wrong fluid can make a difference. Cant think of the correct one right now, but it should be in your manual.

Thank you! Everything felt solid as best I could tell.

I will change the fluid and see what happens.

Something I just thought of, how sensitive is this system to different size tires on the front?

Obviously, I have the same size tires on the truck (265/70r17) but I recently had a flat. All four tires are worn out, but the spare is new.

Do you think that would be a big enough difference to cause issues?

Also, I have 4 brand new 265/75r16s sitting there, ready to go on my new 16" wheels, which are taking longer than expected to arrive.

Not personal experience, but I have been told that will be an issue.