oil specifications

Hi ! to all
I have Ford Explorer 1997 Eddie Bauer all wheels drive
There is little whistling coming around 80 km/hrs, I think that from the rear axle.On the owner book the preconized oil is 75 w 90 synthetic
can I change this oil for 85 W 140 mineral to lower this noise
I have found also the same synthetic oil in transfer case
but in same book the oil suppose to be ATF
THANK YOU for reply to me and explain what oil exactly must be use
on each axle ( front,rear,transfer case,automatic transmission )
for the best results, cheers, IVAN
I want tank you to all of you guys for your lack of interest on my question, very good support untill today nobody reply thank ' s again

synthetic oil is for life of the vehicle. 80w-90 gear lube is o.k. for the back if you want to change. if it is positrac, you may need an addative for the axle. I believe the transfer case is MERCON as is the transmission, not gear lube.