I had my diff replaced last summer (whole thing) with a diff from another automatic 95 Z28... guys at the shop said it was fine, except at first they couldn't get the ABS hooked up right, the sensors weren't set up the same. I thought that was odd, since the diff came out of the same exact car. Now, 6 months later, the diff is whining HORRIBLY again. I haven't taken it in yet, but I'm thinking the shop may have put in a GU2 diff instead of the GU5. Could that be part of the whine? It only started recently, but I can't think of any reason it'd be acting up so soon, unless they installed it wrong. Then again, considering they may have put in the wrong part...

 

The difference between GU5 and GU2 is the gear ratio. GU5 is a 3.23 gear ratio and GU2 is a 2.73 gear ratio. The reason for the whine is you have a problem with your gears. The first thing I would do is look to see if you have enough fluid in the rear. If you don't know how, there is a plug on the side of the housing. It takes a 3/8" drive ratchet to take it off. No socket, but a short extension makes it easier. The fluid should be easy to touch with your pinky finger.

 

There are two different ABS setups used in 1995 and newer years. That's because up through 94, all the cars had ABS, with a single wheel speed sensor on top of the diff housing. In 95, they made traction control an OPTION, and the rear axle for those cars got two wheel speed sensors, located on the ends of the axle on the backing plates.

As clearly explained in the post above, all the V8 rear axle assemblies are the same, other than the gear ratio and the wheel speed sensors. Neither of those would cause your noise problem.

 

to OP,a differential replacement(differential only and using the original gearset) would require the 'lash' to be checked and set/reset.
installing a GU2(2 series) differential in a GU5(3 series)car would require the use of a ring gear 'spacer' to make up for the dimension difference between the 2 different series carriers.
If your car is ABS only,did they remove the 'reluctor' ring off your carrier and install in on the replacement carrier ?
You never stated if your car is ABS or ABS & Tracion Control.
If the 'lash' was not setup and the differential was just 'slapped' in,gear wear/gear whine is probable.