Doc,
I agree with Dave - get somone who is experienced in representing not just on tax matters, but even more specifically on these matters of personal tax at the collection and garnishee of wages point. There are two problems here - one is the problem of whether or not they are corrrect about whether you owe the money - and that likely requires appeals or amended filings - and the second is the jamup on not having paid them yet, so they are taking your money, which they may have lawful authority to do. You need someone who can deal with the first question, as well as negotiate and deal with the second. It is nearly impossible to get your head around sorting out the first problem when the second is occupying your mind and leaving you cashless, so you need some help there.
Two things to bear in mind. First, you can usually negotiate a payment plan with them which is usually fairly brutal but doable (at least, up here in Canada they will often negotiate such a plan). Second, if they think you owe money, they are probably charging penalties and interest on outstanding amounts even as they are garnisheeing your accounts. Those are brutal. If you can borrow the money from a financial institution, even on an interim basis while you sort out what you really owe, you will save yourself a ton of money and heartache.