Rear End Noise

[chrohm]

1986 C10 Silverado bagged truck.

Bought the truck with some rear end whine at 40 mph to 60mph or so only on acceleration. Soon as I let off, sound goes away. Diff fluid is full, but looks a little dirty. I'm pretty sure this is the original rear end with no rebuild. Thinking about taking to the local diff/drive shaft shop. Does this sound like a ring and pinion issue, or do you think I'll need a rebuild? I don't have any clunking or grinding noises.

Thanks