1965 Riviera GS posi chatter

[SLVRARO20]

I have an all original 65 Riv GS with only 24,000 miles on it. It has never been driven hard. When it gets warm and I go around turns slow, I get posi chatter. I think a change of the rear lube with additive would do the trick, but how does one get the old stuff out? No one seems to know, like the local garages, Jiffy Lube, etc. Any ideas? I was thinking some kind of suction device attached to a long hose?

Thanks!

[RivNut]

There's a similar thread on the Riv owner's tech line right now. Check there and see if there's been any suggetions for this problem.