I have a 72 350 auto with 323 gears in back i believe.

after driving on hwy 20 miles or so i come to a stop and turn right i get a chatter type noise in the rear.

i had similar about year ago , changed the fluid and added the gm additive, it went away, but this year it came back, i only put about 400 miles on it and sat till june of this year.

it has 77,000 miles on it.

Take it to a parking lot somewhere and do slow, tight circles in both directions for a few minutes. Might just need to distribute the posi additive after sitting for a long period. Anytime I hear one chattering after a long drive on the highway it is always the posi clutches. Another fluid change with additive might be needed, also, but try the slow circle thing first. After doing the circles get out and drive on the freeway for a few miles, exit, and take a sharp turn in both directions....see if that helps. 

Hey Double A - any benefit from doing the parking lot circles in reverse also?  I recall reading or hearing about adding those circles to the annual maintenance routine.