A lot of times on the expedition the battery cables near the battery get corroded and cause a lack of full power going down to the starter. I have seen this with the ground cable and the positive. And not just the terminal has corrosion on it,but it works its way into the cable and the cable needs to be cut back some and a new end put on it. That is likely what you have here. But FIRST, I would apply my own power and ground to starter with a jumper back from under the vehicle and see if the starter will spin if you bypass the vehicle wiring. You could just have a faulty starter as well, but that will eliminate that.
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