rear clunking?

A couple questions. Is the "clunk" at wheel speed or faster? Does it clunk on acceleration, deceleration or both? With the vehicle stopped, does it clunk when shifting from drive to reverse and back? Please advise.

I would say clunk is at wheel speed. Only hear noise at walking speed or slower. Have heard it when slowing and after releasing the break, as the speed increase clunking stops. No clunking when shifting transmission.

When the noise is present, if you "lightly" step on the brake, does the noise cease? Also, when you step on the brake during a normal stop, does the pedal pulsate up and down?

Hard to answer your first question because it happens infrequently and I have not paid attention to that and the noise appears at such low speeds that when you step on the brake the car stops and the noise ceases. No there is no brake pedal pulsating in normal stops.

Well, the only thing left is the positraction itself. The noise you describe may the clutches in the rear end slipping just a little. This usually occurs when running two different sized tires on the rear. I'm sure your running numerically the same size tires, but the "physical" diameter may different just enough to cause the clutches to jump. You may want to check this. I used to supply tires to a guy with a circle track race car. He ran P235/15 tires on his car and would measure the inflated diameter of each tire. The diametwers would vary as much as 2 1/2" in. on brand new tires. He would run the larger ones on the right side. Let know what you find.

thanks for your analysis. Is this something I can ignore at this time given its sporadic nature?

I wouldn't consider it serious. GM positraction rearends have made noise for eons. It is considered charactoristic of the design, most commonly caused by the wrong additive or lack of. According to your posting, you have an additive so I wouldn't be too concerned.