Dodge Dakota

I have a 2001 Dodge Dakota with 90,000 miles.
My truck started making a loud whining in the rear so I guessed it was the differential and stopped driving it right away. A mechanic checked it out for me and there was no fluid left, it had somehow leaked out. So he took the cover off, refilled the fluid, and resealed it. Then he replaced the gasket. He said the damage to the gears wasn't bad, it just would be a little noisier now. Well, it was doing fine, but today it got real loud again and when I looked under there, I could see the fluid splattered all along the bottom of the truck again. So now I don't know why or if it is leaking or what needs to be done at this point.
Please help.
June 7, 2006.

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