That slop in the rear diff is just that, slop. Its not what is making this popping sound. Focus on the popping noise. Dont create problems that aint really there. It might be time to find a trustwodthy mechanic and take him for a drive, make it make the noise, and see what he thinks? If its as loud and scary as you described, its only going to get worse and reveal itself... eventually... Probably sooner than later.
