Okay, I understand now. On the older cars(like mine)the rear crankshaft seals are considered a last resort unless the engine needs a lot of work anyway. I think anything over 150K to 200K miles is lucky. But they can "seep" for years and years. They usually have to be leaking real bad to warrant replacement.
I pulled the engine out of my 6.3 two years ago to fix the rear seal with 140,000 miles on it. Mine was leaving puddles. Of course I needed to get all the stuff on the engine re-plated and replace some other pieces as well, but I really don't like the old "rope seals". They are a serious pain to do!