Forget the manual, fill it until it leaks out of the fill plug. It's impossible to "overfill" a rear diff...I'm not sure how you would accomplish that even if you tried.

 

Thats what I've always done in the past. But, technically you can overfill it. It'll just leak out the vacuum tube or out the axle seals on the wheels.

 

Absolutely correct. If the op is running the level too low the wheel bearings will be running dry. Not good!

 

:-) My dealership told me to put in what was stated in the manual, which is 2.6 quarts for my 9.75 (non-slip differential I believe.) It did not come out the fill plug hole, so I was I concerned. My contact said that was OK, and just check it every 6 months and confirm it can be felt with the fill plug out. I am taking him at his word, but I guess it wouldn't hurt to put more fluid in.........

 

I was taught to pull the fill plug and put my little finger in the fill hole. If there was fluid on any part of my finger, including the tip of my finger the differential had a sufficient amount of fluid. Filling until the fluid came out of the fill hole was just wasting fluid. One tube of modifier should be more than enough. I would put the modifier in no matter what the fluid says. Except if an aftermarket cover held more fluid. Then a little math would tell you how much you need.