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For the Ford 9 in. rear axle, remove the oil with a suction gun through the filler hole. The lubricant level should be maintained to the bottom of the fill hole with the axle in its normal running position. If lubricant does not appear at the hole when the plug is removed, additional lubricant should be added. Use hypoid gear lubricant SAE 80 or 90.
We used to remove the entire carrier assembly, drop the driveshaft and remove the carrier assembly, there should be a fill plug on the carrier assembly.
Rear differential fluid capacity is 2 1/2 quarts.
There has to be a fill plug on this look on the back side of the housing also.
Here is a picture where most are being relocated