Had the same issue with a Dana 60 I had built for my Challenger

At first I thought it was the diff cover leaking but after checking closely it was leaking from the plug welds where they installed the new axle tubes and running down and dripping off the diff cover.

After some research on fixes and not wanting to pull the rear end back out I mixed up some JB Weld and filled in and covered all the plug welds.
I took my time and made sure to get the JB weld completely inside the plug weld and made it look clean and smooth.

I waited a couple days and painted the areas.

The fix turned out real nice and hasn't leaked since.