Occasional clanking from cone-type Sure Grip 3:55

[skyhawk61]

      Every now and then I get a noise from the cone-type Sure Grip in my 70 Charger R/T that sounds exactly like a tailpipe hitting the bottom of the rear frame rail.
It usually(but not always) occurs during light throttle or off-throttle position, such as when the car rolls up to an intersection. Turning left or right doesn't make the sound appear, or go away, for that manner.  When it does occur, I can "blip" the gas pedal to "shock" the drive line and the noise stops.  The Sure-Grip pulls equally and is otherwise pretty quiet; I sucked out the old fluid, replaced it with Mopar Sure-Grip fluid from a full quart can I bought back in 1969, but their was no change in the "rapping" noise...it still comes and goes. No leaks to speak of, and whether the rear end is hot or cold makes no difference. The diff was in the car when I got it 5 years ago, and it's done this since I've had it. i pulled the brake drums, checked the axle bearings, installed new shoes and brake springs to eliminate shoe "snap."...No change. I installed a new drive shaft and U-joints...still no change. I checked the pipes and muffler clearance just to be sure...they are OK.
Any ideas on where else to look?  Thanks for your help

[Cooter]

Sounds like it's time for that Sure Grip to have new bearings installed. Afraid it's time to come apart, before it comes apart...