I am also having a similar problem with an assembly I have. Parts that are not grounded or fully constrained act as though they are grounded and will not move. I have tried deleting one of the faulty parts and then re-contraining it and I still have the same problem, so I don't think this is the issue.
Check for "Sick" constraints in your IAM file. To the right of your Joints and Constraints icons are three icons, if the middle one (highlighted in yellow) is not grey'd out, you have sick constraints.
Software has changed considerable from 2008, also make sure you are current on all SP's and hot-fixes for your product.
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