In my experience, weakness in the hind legs is less of a specific symptom but
rather an overall indicator of health.  In other words, one of the first signs
of a ferret being weak seems to be difficulty with the rear legs - it's a very
non-specific sign which can be caused by almost *anything* that makes the poor
furball weak.  All just my unofficial opinion of course.
 
Bill
[Posted in FML issue 1133]