2wd I have a loose clunk in RR wheel and center at diff, diff has sludge leak? on 1991 Ford Aerostar

I get a loose feeling clunk when going over bumps and potholes, I have high pitched whine on turn. Thick sludge is present on driveline/ujoint side of diff housing. Previous owner replaced broken trans with rebuilt one from wreaking yard before I purchased it. It shifts smooth but now diff sounds like axle on right is loose and sloppy and wheel is following suit as well can hear clunk at diff housing as well. No axle seal leaks at the wheel bearings can be found by my inspection. What should I do? Have done similar repair on my Toy 4Runner but only replaced bearings and seals with inspection of diff showed low fluid and no metal shavings present in drained fluid.

Asked by for the 1991 Ford Aerostar
You need to check for metal shavings in the fluid on this one. Have you ever serviced the rear end, and the wrecking yard rebuilt this one ? I wonder what was put in it to keep it quiet, and if the pinion seal is bad, you could have lost some of your fluid there. Do an inspection.
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