Normal rear end behavior?

[Nixon1]

Lately I've noticed something.. During acceleration, ONLY in 2nd gear, there's a whine...sounds like a differential whine....goes away once it hits 3rd gear. Also...decellerating, RIGHT when it's in Overdrive and the tranny downshifts to 3rd gear (which is a very very rough downshift by the way), there's another loud whine that sounds like the rear. Is this normal behavior? I've got 4.10 gears in it, and the differential cover was warped...I was leaking gear lube for a little bit, then the tranny shop serviced it but the jackasses didnt replace the leaking cover, so when they put it back on it opened up and leaked like a MOTHER...had to have it serviced all over again..and plus the scumbags didnt put friction modifier in it. I'm just wondering if what I described sounds like gear wear, possibly caused by a low gear lube fluid level? Thanks everyone!