rear diff whinning ?

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Well, first of all, the factoy spec fluid is 80W90. Secondly, did you put in 4oz. of friction modifier?? For trac loc axles need this. Try that first....if it doesn't help, I vote for bearings as well. Bearings are cheap, labor is double.
 
I just use Valvoline 80W90. No need for synthetic. Bearing kits can be purchased from www.50resto.com I am sure. See I just take my car to my rear end shop and they just do it up when I have a problem.

I look at the manual , saying I should use 80w/140 or something like that. I bought RP from americanmuscle.com and 80w/90 is only weight they carry. when did ford change the fluid spec ?