Loud Clunking Noise

I was having some bearing and gear problems(lots of noise) with my Challenger after I purchased it. It had been sitting since 1981 or so, so I thought perhaps the bearings were having issues after sitting for over 20 years. Not having the tools to do it, I took it to a customer of mine at the time that did diffs., among other automotive work. For whatever the reason, I never was charged for the labour, only the parts. I thanked him, and drove the car home. No issues, no noise. I went to the local hangout here that night, and when I was backing the car up, it sounded like the whole rear end was ready to fall out. The next day, I was speaking with a fellow that worked for a company that only did diffs and he said to get rid of all the oil in the diff, go to my local GM dealer, buy 3 quarts of diff fluid and 2 bottles of GM posi additive. Followed his advise, put the fluid and additive in and like you PROBLEM GONE. I never did tell my customer of the problem, because of the no labour charge, but needless to say, I never sent anyone his way for diff work. Diff by the way was the 489 case, 3:55 Posi.