Yep, I'd put in the correct weight. Don't know what would happen, but I would not want to find out if you left the 90 in. Besides, there has to be a reason to spec for "synthetic 75w140". I think what you read was for the FRONT axle. I thought it was 75W90 however.....????
Anyway, I changed my diff fluids about 4 months ago and found that Amsoil from GI Joes had the same specs that the Ford stuff did for only $12.99/qt for the 140 and $8.99/qt for the 90. The dealer wanted $19.95/15.95 respectively........ I got the Amsoil stuff, but did buy the friction modifier from Ford for only $4.95.
So far so good. I tow a #4500 TT as well too.