You have to isolate the components to see where your problem may be. Check the rear bearing by removing the 1/2 shaft so you can rotate them by hand without any interference from the u-joints and differential. rotate the rotor by hand and see/feel for any rough spots or noise. Check the end play in the spindle bearigns with a dial indicator mounted against the spindle nut. Push/pull and and measure the play-it should be under .005" preferably .001-.003" Ifthey are smooth and have the required endpay look at the end playin the rear end yokes. They most likely will have play in them, buthow much? Over .050" and you should start to be concerned because the metal is in the oil. If there is over .070" I would remove the differential and open up for a look.
It sounds like the pinion gear bearing and or the R&P the way you describe it. With the car jacked up you can get the differential out. Rebuilding it requires special tools and knowledge-but is a job a hands on guy can do.
A vette shop charges about $500 to do a general rebuild if you bring them the differential. To remove it and rebuild it will cost much more-depending where you live. I've heard of shops removing and rebuilding them for a little as $800 and as high as $2,000. It cost me about $600 in parts to go through them completely,that includes new gears.
Good luck, read up on it and shop around if you're not doing the work.
