I'd get a reman Napa Gold axle and call it a day. They seem to be the best ones besides OEM. Never tried a used one since I don't know how it was treated beforehand. Others may have better info on a used one.

Question is when was the last time the fluid/filter done? At that mileage I'd do a pan drop/filter change and install a Trans-Go shift kit while the pan is off. It will help the trans live longer. Some throw a quart of Lucas in it as well at this point.