seriously with those sort of mile on the car it cant be anything bad, even if it has been driven hard. but the report the OP mentions says it been cared for etc.

Also the rear is a torsen so wont need a re-build etc like clutch type. Also if the bearing are 'gone' for whatever reason, then a kit is like $100 from rock auto for carrier, axle and pinion bearings (5) and the 2 seals. Get your dad's pal to rebuild it and your good. I honestly dont think they will have gone yet uness the rear was ran out of oil, which is unlikely on a 41K mile car

I actually think the problem maybe the backlash of the diff gears themselves, have these been atlered for aftermarket ones ever?? also some of the later f-bodies did have notoriously tight rears and whined made noise.

Dude the $1400 quote is a joke, i would have made a vulgar jesture in the shop then walked out, but thats me. You can by a good full rear axle complete for $400-500 from salvage yards, or heck just add another $600 to the $1400 and get a moser 12 bolt

I think it would be good to go for a drive with your dad's mate and see what he thinks it is, or a pal who is good with cars etc first

 

Just a FYI on Bearings. The most likely cause of a failed bearing is how it is used but automotive parts are almost always given to the lowest bidder. As a general rule bearings are made to a CPK of 1.66 or 5 sigma. That means there is almost a 1 in a million chance of a defect. But a defect would be a part that failed within the warranty period. Bearing quality can vary greatly within the acceptable range defined by GM/Ford who ever. The more variation allowed the cheaper the bearing. Make it cheap, fast and almost defect free. Very few of these bearing are still made in the USA. Ya I know a lot of them say made in USA when they should say assembled and boxed in the USA. We do make bearing for safety application that are as close to 100% defect free as possible but those go in jet engines and pumps for nuclear reactors. The cost of those types of bearing would send most of us into sticker shock.

So in most of cases like this it ends up being a combination of a not so great bearing in a car that gets used hard. The 1 in a million defects also assumes everyone involved follows all the process everyday. Humans always screw things up then try to hide that fact they screwed up.

 

I had my differential rebuilt for $450, including parts. This did NOT include new gears or axle bearings (as I had already replaced them)...but I had a similar problem with noise from my rear end, and it wasn't the taco bell I had been eating.

It *may* be worth replacing the axle bearings and seals before you dig into the differential. Axle seals involve removing the differential cover, getting showered in gear oil, removing a small clip on the axle, and pulling them out from the side of the car. You will also need to remove the brake caliper to do this. I used a slide hammer to remove and install the bearings but I don't know if that was really the right way. On the plus side, axle bearings are cheap and its something you could try yourself, first.