Q&A

 Master

That engine uses a timing chain to time the cam and crankshaft sprockets, the balance shaft is driven directly by gear to gear contact and should never go out of time like a n engine that would time the balance shaft with a separate timing belt.I would get a second opinion and hope the imbalance is sometime more simple. With a professional grade scan tool look for misfire counts and anything in Mode 6 that may not yet be severe enough to set a code. May be first get your mechanic to try this.
There is a professional service called Identafix. http://www.identifix.com/info/about_us.htm They only deal with professional repair shops. They are in the Minnesota 651-633-8007. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people. If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base. Wheel imbalance is normally most noticeable at 50 to 60mph perhaps a wheel weight fell off.