Yesterday I noticed that my rear differential cover was wet. A drop of oil hangs on the bottom edge of the differential and a small spot on the pavement. After doing a research online, my first guess is that overfilled oil oozes from the fill plug. I plan to take out the fill plug and check the fluid level. Here comes the question:
1. People say it takes a 3/8 hex key to unscrew the fill plug (as well the draining plug), however, my fill plug has a square hole on it and the hole looks bigger than 3/8. Is this the way it should be?
2. It is required to tight the fill plug by 33 N-m (24 lb-ft). I dont have a torque wrench. How large is 24 lb-ft? Is a good tightening with the hex key enough?
Sorry I dont have any pictures. My situation is similar to this one:
Rear Diff Seal leak?
I borrow his picture, I'll take some pictures of my own when it stops raining:

 

I doubt that it leaks around the plug,usually a seal,or the plug is not tight.
snug+ should hold it, 24 isn't much,

 

3/8" ratchet ... Check the plug gasket? Tighten same as oil pan drain plug.

 

[QUOTE=hummerz;298221]3/8" ratchet ... Check the plug gasket? Tighten same as oil pan drain plug.

Thank you very much happythree and hummerz!
Over the weekend, it seems that it stops leaking. I plan to clean the differential cover and drive a little bit to see if it still leaks. Later I'll check the fluid level. I'll update.
Is there any chance that heat can cause a small leak? The day I found it leaking was kind of hot, like 91 degree. The weekend it's raining and cool, and the leak seem stops.

 

I think I located the leak. It's the fill plug. I cleaned the differential case (really dirty), see the first picture below. About 1 hour later, I checked again and I saw a clear oil path from the fill plug, shown in the second picture.

I tried to tight the fill plug with a 3/8 ratchet but I found it was already very tight, I can not tight the screw anymore...please see the third picture, hands up if you think I rotated the screw a little bit, lol. I clean the area again and wait to see if there is another oil path tomorrow.

 

Did you check the fluid level? I would do that before continuing to drive it.

 

Thanks for reminding. I didn't check it. I think since the fluid still leaks from the fill plug, the fluid level should be above the plug. Plus, I'm worry about that once I take of the fill plug, I can not tight it as much as it is now.
I also worry about the gasket of the fill plug is broken and causes the leak. Is there any chance you know which kind of gasket should I looking for in a part store?

 

so just pull the fill plug and let it drain. somehow it got over full?
you could use pipe thread dope or Teflon tape on the threads.

when you fill, it should drain out the fill hole then plug it.
I never saw a gasket on a fill plug,just threads usually seal it when tight, use tape

 

When I got the truck I took it to the dealer and changed all the fluids (both differentials, transmission and transfer case). I'm not sure if they overfilled it or not. It seems so.
I remember seeing somewhere both the fill plug and drain plug are sealed by gasket, maybe I remember incorrectly or it's about other GM truck. It's good to know there are no gaskets, so nothing can be broken. I'll check the fluid level, I'm not comfortable driving it without doing the check.

 

if its leaking out the fill plug,its "filled" usually when filled with new juice it runs out the fill hole ,when it stops,you plug it