95 explorer whining rear end

[vintagecar]

I just got a 95 ford explorer. There is a whine coming from the rear end. I've done my searches and reading. I'm taking a long shot and going to change the fluid first, to see if it will reduce the whine. I went to get gear oil, but there are various kinds... 75/90, 80/90, 85/140, 75/140 synthetic. I read there was a TSB for 97 up to 200x to change from 75/90 to 75/140 synthetic. Mine is a 95 and it whines, what gear oil should I try to see if I can reduce the whine?

 

[dogfriend]

140 weight is more viscous (thicker) than 80 or 90, so it would have more of an effect on quieting the gear noise.

My 97 w/ 4:10 axles came with 75W140 in the rear as factory fill - I replaced it with AMSOil Syn of the same grade several years ago.