Rear derailleurs do wear out at the pivot bolt- they can develop enough play to be hesitant to shift, especially shifting to a larger cog.
Any Shimano rear derailleur would only work if it has a long enough cage to wrap all that chain from the wide range of gearing usually found on MTB's. You are unlikely to find a long cage (SGS) on a Ultegra RD.
I would suggest a new Deore rear derailleur. Here's one for for $19, a bargain!
http://www.jensonusa.com/store/produ...x?SSAID=176195
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Last edited by kenhill3; 11-28-09 at 10:17 AM.