The beast has just developed a "ping" from the driveline when shifting from neutral into forward or reverse (auto) at standstill. It has also just developed an occasional "scraping" sound which is audible at low forward speeds, but then seems to just go away. I thought it was a stone chip caught up in a rear disc/spash shield, but nothing there when I looked.

The GM '81 workshop manual sez likely cause of these are a "loose companion flange," (gm fix = tighten bolts) and "slinger companion flange or end yoke rubbing on rear axle carrier" (gm fix = straighten slinger.) Trouble is the book doesn't explain or show what these are or where they live ! The end yoke I can figure, but where and what are the "companion flange" and the "slinger" ????

Confused, yet again... Chris :-)
(cos I don't know how to do the fancy smileys that you   
guys use.)

Sounds more like a bad u-joint, to me!
The "companion flange" is the part that the half-shaft attatches to on the trailing arm. It has four bolts that hold it to the spindle, and the half-shaft attatches to it with straps & bolts.
I think the half-shaft u-joints are the actual source of the noise.
btw.....it's usually best to replace all (6) of the u-joints at the same time. They all have the same # miles, so if one is taking a dump, the others are not far behind. It's the old 'Better safe than sorry' routine!

Sounds like Apple is right on again, sounds like the universals are as dry as the Texas landscape and the sounds you are hearing are the harmonic vibrasions travaling up the drive or half shaft of the needle bearings being ground to dust. anips38558.6465972222

I'm with apple and anipe.  check the u joints on the half shafts too.

Thanx for the info. I think both universals on the left half shaft have previously been done, as they are both fitted with grease nipples whereas the right side ones are not. As far as I can make out originals do not have grease nipples - can you confirm this ?

Which now leaves me with just 3 questions...
(1) get new universals with or without grease nipples ?
(2) what on earth is a slinger ?
(3) any recommendations on where to get new universals ?
(4) where is Texas ?
(5) the wee bolts which hold the straps look like they
    have kinda unusal "star" heads. Is this right ? do
    they need a special socket ? or do my eyes need
    adjusting ?

Never could count.

The original u-joints do not have greas "zerks"(or nipples).
My personal preference is to use ones without; they are stronger without holes going thru them.
A "slinger" is a sheet-metal shield on the yoke. It's purpose is to keep dirt & such away from the seals. It's attatched to the yoke, and sometimes gets bent to where it will contact the diff. housing, making a noise.
I'm not sure where you would get the u-joints in your neck of the woods, but most of the mail-order 'Vette suppliers will have them.(Mid-Am.-Eckler's-Bairs, etc.)
Texas is right here! (Lower-middle-USA)
The bolts for the u-joints are 1/4" 12 point. They can be very stubborn sometimes to get out. Soak them with penetrating oil for a few days before trying to remove. They can be replaced with regular-headed bolts, if you wish, but they are an unusual size, and may be hard to find.
Adams' Apple38559.2121064815

The 4X4 guys use the joints that you can grease through the caps, altought the joints without the Zerk's are stronger I can't ever recall ever breaking a cross with a zerk, burned em up, loose nut or bolt, but never broke one

I can't believe it has been 18 months since I first posted this !!!

I have been really really bad... I have only just today checked out the u-joints, and sure enough they were history. Although running around on them for so long will have taken its toll also. All the needles missing from one, and half of them from its opposite half. Two caps in the process of self destructing. I am soooooo lucky that it has not let go on me !

I just wanted to add the outcome (albeit very delayed) to this post so that anyone else who reads the post at a later date will know the outcome.

The amazing thing is how it started off very suddenly as an oh so loud "scraping" noise, just like when a stone has got caught between a brake disk and its weather shield. Then within days it had reduced to a much less noticeable "knock" only when in a right hand turn and applying power. (Yes it was the right hand side half shaft u-joints.) I can only guess the scraping was sound of the needle rollers being tortured out of their home and onto the road.

Why did I take so long to do it ?   (a) stupidity and (b) I don't have a pit, and so I kept putting it off.

If you ever get this symptom, don't delay sorting it like I did..         You may not be so lucky !

You are very lucky.  I had an incident back in 1971 when one of  the center u-joint in driveshaft of my 63 Impala SS went south and the drive shaft went banging around in the X frame and then beat the heck out of the floor pan before I could stop.  It wasn't pretty.  Ever since then I inspect the u-joints in my cars and lube them annually.  It might be a little overkill but it gives me piece of mind.