Before swapping out the diff because of the excessive play seen when
turning the wheel, you may wish to try replacing the thrust washers.
This operation is described in Porter's book & others as a cure for the
"Dreaded rear axle clunk".
It involves replacing 2 cupped copper washers & 2 fiber spacers that are
associated with the sun gears (& planet gears?) as well as replacing the
cross shaft. This is not a particularly difficult job, but does require
the partial removal of 1 axle. I did this as a tech session for our car
club using a rear out of a car & up on a bench, took about 25 min. Allow
2-3 hours in a car, as you will have to drain the gear oil, remove the
brake backing plate from the driver's side to partially pull the axle.
Cost of parts was aroud $75.00 US as I recall.
You can drain the diff, remove the cover & do some inspection first to
see if you can determine where to play comes from before making a
commitment. Laurie, Ed, & others may give you additional/better info.
On the other hand, if you can source a used rear that has little play
for a reasonable price, do the swap, then re-do the original unit for
resale or a known, good spare for the future.