For a year or so, I've had a popping noise in my '66 Mustang's differential or rear u-joint. Can't tell which. Several years ago, I had a limited-slip conversion done. The special additive was used. After that, I had to have one axle replaced because it had too much "run-out". But, I don't think the popping noise started until a couple of years later. The rear u-joint is part of an add-on releasing mechanism that allows you to disengage the driveshaft when you want to tow the Mustang. Any hints?