Axle Seal Leaking. Help please.

I kept smelling rear end fluid for the last few days.
Sure enough, Left rear wheel is covered in oil.
Never done one on these trucks. I did manage to get the diff fill plug broken free. It was stuck pretty good. I was over 1 qt. low. I added some until I can get it fixed next week.
Weird thing is the diff made a hiss sound when I cracked the bolt open.
So, I pulled the diff breather and checked it. It works fine. You can blow air through it.
Is it normal to hear some air escape (or be sucked in) when opening the diff fill plug?

How hard of a job is it to do these seals?
The parts were not expensive...$14.xx
But Labor was almost $500.00 for ONE SIDE?.....

Figure I might try this myself. But never done one before?

 

Not hard, but you have to be willing to get dirty. Check the "removing rear elocker diff" thread, you only need to do about 1/4 of the steps outlined there. My diff had pressurized air too, look into rerouting breather and putting an open breather (2 way). You can do all this for less than $500 worth of time.