I read through the past article and through others questions on this topic, but I am getting conflicting answers. I had a flat tire and the tire company put two new tires on the rear of my Explorer. They were the same make and model as the tires on the front. The mechanic told me that I should have put 4 new tires on the car because I can start having issues with the drive train. I went back to the tire company to get the two front tires replaced and they told me the mechanic was wrong. As long as the tread was the same on the front tires and the tread was the same on the back tires it wouldn't matter. So now I don't know what to do. I will say I did start hearing a high pitch noise when I go at highway speeds. Who is right here?
Is your Explorer 2 or 4 wheel drive?
If 2 wheel drive there is no issue at all - the two ends are separate.
If 4 wheel drive there is a benefit to swapping all at once, as it reduces any differential slip issues through differing tire diameters, but you can have worse issues from having a tire running at too low pressure. It's not the tread, specifically, it's the effective diameter.
Have a read of this question for some extra info.