Last week when I serviced my Blazer, I decided to check all my cases. Front, transfer all good, then I get to the rear differential. I checked it when I first bought it last March and the oil level was fine. However this time I couldn't get anything on my finger. I ended up putting over half of my Castrol 80-90 bottle in and was alarmed it took so much. Where I park everyday, there is absolutely no puddles or wet spots underneath the engine or any of the cases. That's when I noticed over an inch of oil/dirt crud caked around the pressure relief line. After knocking all that crud off (and inevitably having some drop on my face and eyes) I took the hose off, was able to blow through it, pulled the dust cap and it still worked fine. I ran a zip tie through the nipple on the rear case. Didn't feel anything. What puzzles me is the case is perfectly dry on the back and bottom. Has anyone else ran into this problem? Is that what happens when that pressure relief line plugs up? I was always told to fill until you have oil run out of the middle plug.
I love my Blazer, but I don't think I'll ever buy another one. Just seems like a constant train of little problems. (I know she's 18 years old...she seems to be burning/losing about a quart of oil per month at 153k.)

 

I know those diff. covers are known to rust and get really thin and oil seeps out. I just replaced my cover and it was only about $12 on Amazon.

 

The seals can leak at that age and mileage. You mentioned that the case is dry on the back and bottom. How about the pinion seal on the front? There are also axle seals on both ends of the axle tubes. Check for oil at those places. You're right to think that there are only a few places that oil can leak from.

Until you find and repair the leak don't let it run dry. 3 relatively inexpensive seals can save expensive, pressed in bearings and gears. Timken manufactures top quality replacement seals.

 

That's what gets me-the pinion seal is completely dry. It's gotta be something on top-I need to drag myself yet again underneath with a scraper and light to investigate. Better check the oil level again. Thanks for the tips.

 

Are the axle seals on each end of the axle clean and dry as well? It is quite possible for the axle shaft seals to wear out and start to weep.