No "common leak issues" with the 4T80E.
Don't believe that one.
Left side axle shaft leaks at the third clutch housing bushing BEHIND the axle seal....
It leaks LIKE it's the seal but it's far worse.
When I discovered this issue, at least two years back, there weren't any real repair solutions for this THEN.
Now there are a few repair options with a different sized tripot that fits into a redesigned bearing or bushing.....
Search 3rd clutch housing bushing or my then thread "4t80e Woes"
The leak is probably the bushing which leaks while the vehicle is running. Possible a line but highly unlikely.......
Good luck.
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http://www.fitzall.com/94125LB.php
Here's one possible solution......