Toyota Repair: pinion bearings - rear differential, pinion bearings, rear differential

Question
I have a 1989 Supra that I recently got an oil change on. While there the technician advised that I should have my rear differential oil changed.  I agreed to do that after he advised that the oil was black and it was supposed to be yellow.  Following the change in oil the rear area of my car has been leaking.  It dripped for two weeks and then I returned it to the store.  They filled it up again and said that what they touched was not leaking.  They advised that it was the rear differential gasket and that it was just conicidential that it happedn to leak after they filled it up.  The car now has a very loud whining/droning sound when I am decelerating in the rear.  I took it to a transmission place and now they are telling me that I need new bearings.  Can you explain this system to me.  Do you think I have a leg to stand on with regards to the leakage and who should pay for these repairs.  Could lack of oil do the damage they are saying or would it be wear and tear.

Thanks for your help

Answer
Differentials on Toyotas normally don't cause a gear noise unless it was run out of oil, which I believe is what happened here, but why?
Here is what I think happened, the mechanic that changed the gear oil probably forgot to tighten the fill or drain plug causing most of the oil to leak out, I don't buy the story about the differential gasket and it being coincidental, I've seen this happen way to many times to believe that story, suppose that it was leaking when they changed the gear oil, then why didn't they tell you that you should have the gasket replaced then, these kind of leaks don't just happen overnight, like you said it dripped for two weeks right after they changed the oil, they are lying to you and are trying to cover their mistake, did you ask the other shop if they believe the gasket was leaking?
When the oil was filled back up and the area cleaned and the gasket was the cause of the leak it should still be leaking(dripping ) right now, if not it was the drain or fill plug that wasn't tight.