sounds like you've got a lot of backlash in the cv's/diff
it's also possible that due to the age of the rear end there is a bit of play in the shafts as well
the only way to sort it is to bite the bullet and get some new cv's
they are about £50 each last time i looked but if you can afford to have the car off the road, pull the driveshafts out of the back end and see how much slop there is in the actual shaft to inner cv ring
if there is any present then you might be looking at new shafts as well as new cv's