My 89 camaro has been having some trouble with certain gears and/or differential problems. There is this horrible grinding/growling noise coming from the rear end of the car: when I'm in overdrive, sometimes when I turn, in first when I accelerate aggressively, and when I go fast enough in reverse. No growling or grinding when I coast or let off the pedal, only when I accelerate. I took the diff. cover off and took a look, nothing seems to be wrong, just normal wear on the gears, but I find that the diff gets a little warm when I go for a drive(5-10 mins), and the fluid seemed burnt when I took the cover off.
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-and I am having some seriously erratic shifting. I will sometimes think I'm in second when I'm actually in first, or vice versa with third and second.
When from a cold start and drive, at 60 MPH it's at 3500rpm in third(growling/grinding ALWAYS in O/D so I just put it in third) when it usually was a little less than 3000rpm(Is now once it's warmed up)
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All this started one day when I went for maybe a 40 minute drive and at the 30 min mark I just heard this clunk noise and then all of it happened. Once I stopped at a light, it was almost like the rear brakes were catching themselves when I accelerated. Same thing for when I went in reverse. Since then only the things mentioned in the previous paragraphs have been happening.
Has anyone experienced this before? I can't tell if this is a differential problem or a transmission problem since the noise is coming from the rear but only in certain gears.

Edit: trans is stock 700R4
HELP!!

My guess is something let go in the differential; was there any slop in the pinion when you inspected it?

The pinion seemed fine, no play except while turning/spinning it(when switching directions there was a very small bit, but it didn't seem like that would have an effect) Otherwise it seemed pretty solid.
But I'm starting to think that it's my transmission that is causing everything.

If you can't pin it down, take it to a good shop to diagnose; they'll put it on a rack and run it while listening to the different components with a stethoscope and pin it down that way.