Sounds like the pivot bolt is loose is all. Side to side play can be caused by this. You need two Torx bits, T10 for the pivot and T-6 for the majority of handle and clip screws. It does not sound severe at all, just needs adjusting.
Most pivot based knives have a sweet spot; find it, set the tension to your liking and you're done. Sometimes a drop of BLUE Loc-Tite can help to keep the bolt at that sweet spot setting. The BLUE 242 formula allows the bond to be broken if necessary, DO NOT use the red version please. (Red sets much harder, and can prove to be a bear to break that same bond.)
If the blade has actually scored the liners, (from the shaking you've described) and you're concerned about that...as a warranty issue, it may require the knife to be mailed up to Benchmade. Their number is: 800-800-7427, and in case that's what they want you to do, here the link to the warranty repair form:
http://www.benchmade.com/images/pdf/Warranty_Service_Letter.pdf
Good luck, and welcome to the Benchmade sub-forum.