OK... ran the 69 to PIR (In PDX) for the every-Wednesday-during-the-Summer-drive-in. Five hundred cars or so there.

Let it set there for 3 hours or so. Drove back (This is roughly a ten mile or so trip.) No problems, no noises.

Dropped my wife off at the front door, in preparation for putting it back into the shop. I put it into reverse, started backing up, and sounded like a sledge hammer was banging my driver side rear wheel. It felt like the tire was square.

Drove it backwards about 6 feet (Bang, clang; bang, clang) and stopped.

Got a flash light. Looked at everything, nothing looked out of place. Differential was very warm, if not hot. No leaks. Driveshaft tight.

Let the car coast forward 3 or four feet (Clang, bang) and let it set there. The "clang, bang" part seemed to be coming from the driver side rear.

I grabbed the rear wheel and jerked it, in and out, and there was no movement of any kind.

So, I call one of my buddies who owned a mechanic shop back in the 70's and asked him to come over and check it out.

So, this morning, he comes by. I move the car and...

... nothing. It didn't make a sound.

We drove the car. Again, nothing. We drove it for around 5 miles of stop and go... nothing.

Now, I haven't pulled the drum to look at anything. I intend doing that, and I intend to replace the differential fluid (It, most likely, is at least 10 years old.)

Not a sound now. This isn't the stock rear end by any means. And, I'm not sure if it's even a GM (as opposed to after market) rear end (any ideas where to look?) it's got an A-body set up for the links above the differential... something like this:

http://www.moserengineering.com/Images/Housings/images/12bolth_02.jpg

It has very high, but unknown, numerical gears. What do I do, short of shooting it?

It's just your limited slip clutches chattering when hot.
Get some GM Limited Slip (Posi-traction) Axle Lubricant Additive and you should be good to go.
http://paceperformance.com/images/products/manufacturer/GM/images/1052358.gif

Great! Do I have to go to a GM dealer to get this, or does NAPA and so forth carry it?

Previous posts have said to use only the GM additive. It's only available from GM dealers (and Rick's). I've been experiencing the same symptoms with a stock rear (140k) and about to add some of this lube before a big drive this weekend. We'll see what happens.

Great! Do I have to go to a GM dealer to get this, or does NAPA and so forth carry it?
I have used the Sta Lube version from NAPA with no problems, but others swear by the original GM version. If you can get it from the GM dealer easily, I would go that route.

Did the deed... absolutely nothing in the fluid chunk or shavings-wise... looks like an Eaton, I think... (I have no real idea of the gears, except it takes an awful lot of revs to run this at freeway speeds)

http://a414.ac-images.myspacecdn.com/images01/13/l_5e8d5616a1be5447f5e88f6e0ae97e6d.jpg

And the mystery rear end?

http://a345.ac-images.myspacecdn.com/images01/112/l_aecaf8abfd1aacaa01014759169e3c00.jpg

Here's both sides of the internals:

http://a158.ac-images.myspacecdn.com/images01/89/l_d46ee7789c346bab54e03cef17a98cdd.jpg

And this:

http://a962.ac-images.myspacecdn.com/images01/105/l_d2f4d0184835b0bde955bd7e27b14c79.jpg

Any ideas?

I copied all of the numbers down I could find in there...

And I couldn't find any numbers on the outside of the case...

These were in the differential:

On the pinion: 3853008-11 3|9 or 9|9 11 65 with a B in a circle painted on it. Also, the 3|9 or 9|9 has a painted "46" over it.

The spool or whatever had the following number: E0-3014PMI (or 1)

The gear was bolted to a piece with the code "A2C" stamped on the edge.

Any ideas?

Count the teeth on each and divide ring gear by pinion gear to determine your ratio.

By the way, the GM posi additive cured all my bad noises!

Its a chevelle rear.