Thanks for the advice - it has been good. That thunk sounds exactly like the same sound in the back of my '69 1600, bought new for $2850. I still regret letting it go at 185,000 miles. And the people who sold me the 55,000 mile warranty made a big profit on it - they've paid out a total of some 600 bucks.
The threads here on high mileage problems are very useful too - I wish I'd found y'all earlier.