If it's clunking when making turns you may want to add some differential additive assuming it's a Suregrip. [/quote]

I agree that is usually the "fix" for the clutch type Suregrip. I don't know what the dealer was doing back then for a repair, but they would keep the car for days, waiting on parts!
I put an "open" rear end in it for while, and then a 3.23 Suregrip from a 69 RR (489 case) which is still in the car today. It will need a rebuild or replace though, as it doesn't always pull with both tires.

Thanks,
Mark