When you changed the fluid did you put any additive in ? The rear takes 75/14 and I always put in friction modifier as well. It could be salvageable. take the diff cover off and check the gears and look for discoloration and funky wear patterns. also look at the axle bearings as well.
Could be the axle bearings going south on you. With the wheels off , take the rotors off and check how much play is in the axles also look for signs of leaks.
With 200k and if you aren't comfortable pulling the guts out a used unit probably would be best. Also you should have an idea of how much millage is on the used unit.
Now from my experience getting used units you will probably need to swap over all your brake stuff- the backing plates unbolt as a unit-you may have to swap out the emergency brake shoes. And you might have to replace the axle and pinion seals. Now the pinion seal is tricky, after you replace the seal and you put the yoke back on ( its the part that the rear u joint goes into)
you have to be careful not to over tighten the nut otherwise you wipe out the pinion bearing.