Rear diff fluid capacity with lift

I was changing my rear diff fluid today, but unfortunately my driveway or anywhere around my house is not level, probably a few degrees. In addition I have the general spring HD springs, which added 2" of rear lift, plus a 1" front lift.
I ended up adding more than I thought I needed just so I could drive to find a flatter location.
I poured 4 quarts total of 75w90 in, and while shining a flashlight in the fill plug I could see the level just below the plug. I checked the owners manual and saw I should only have 3.12 quarts for the rear, so definitely overfilled.

My question is, with the truck modified, is it still accurate to drive to a level place and just pop the fill plug to drain the excess?

 

I always just filled mine till it starts coming out

Your lift will not alter the capacity of the diff. It may put your diff at a different angle, altering the amount of oil you can put in before it starts overflowing.

 

Since the manual said 3.12 quarts, I just assumed the 4 quarts I put in is way too much? Whats the margin for overfill/underfill?

 

I would drain a bit out. I could possibly see diff fluid becoming aerated if there was too much contact with the gears. At least i think that's a possibility.

 

Definitely let some out. Go find a level area, open the fill plug, and all the excess will drain out.

 

It may start coming out your axle seals into the rear drums
Or clog your rear differential breather.
I'd find level ground and drop the plug again.

 

what the manual says

 

assuming your front and rear tires are of equal diameter?
place a torpedo level to the frame just below drivers door.
it will show which is higher or lower, front or rear?
also assuming that the driver side and passenger sides are equally level, on the same plane. (the level placed on rear axle will display that)
you don't have to drive far, just forward or back enough onto plates or blocks of wood to level the low points (unknown how steep your driveway is?)...

when level?
remove the fill/overflow and let it out...

 

fill till it comes out is fine. It won't push past the axle seals unless your vent is clogged.