First off, I'm not an expert, as I've only had my cresteds about a month. But the new Rhacodactylus book has a section on floppy-tail, and I agree that it looks like this is what you're dealing with.
I quote from the book: "This syndrome once it has occurred is not treatable. On the other hand, floppy-tail will usually not affect the gecko's health or its ability to reproduce. To prevent floppy-tail, provide adequate amounts of calcium and D3 and landscape your enclosre with bark and plants to reduce the probability of your geckos spending extended amounts of time on vivarium walls."
When the gecko spends all it's time hanging on the walls, gravity on the tail puts a lot of pressure on the pelvic structure, which can initiate this problem. The book notes that this also occurs in the wild, so it may not necessarily be due to an error on your part. Just something to consider. I'd highly recommend spending the money on the book - Rhacodactylus - The Complete Guide to their Selection and Care, by de Vosjoli, Fast, and Repashy. I got mine at giantgeckos.com. Good luck.
Jay
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1.0 Ball Python
0.0.1 Corn Snake
1.0 Leopard Gecko
1.1 Crested Geckos
1.0 Red Eyed Tree Frog
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"The more people I meet, the more I like animals."