The EX calls for 75-140 in the rear, with 4oz of friction modifier. The rear diff on this truck is known to have a harmonic whine under load. I did a gear oil change with Supertech 75-140 and Motorcraft's friction modifier. Noise was still present. I guess im out of luck getting that thing to quiet down. However, the front calls for 80-90, which I changed out with Supertech 75-90. After about 2 months or so, I am now getting noise from the front end under load, only at certain speeds. This I am assuming is due to the way Ford's advancetrac operates, causing the front diff to operate only during certain times. Is this correct?? Also, would it be an option to go thicker in the front, say to 75-140 to attempt to quiet it down, or would there be adverse effects due to using a thicker gear oil??