JeffBall

My mustang is pushing 210K miles, my reverse is making a loud grinding sound, it seems to be doing it only at low speeds, today i replaced the rear differential fluid and the rear gears seemed fine, and still was the exact same sound when done with that, so It has to be something in my transmission, none of my other gears make this sound except reverse, a maybe 7k miles ago i changed my transmission fluid to royal purple. My transmission fluid is still full, and still the same feel as when i put it in, same color, same smell. Could it be possible my reverse is just going out? There are kits on ebay for around $120 is that reasonable?