The IRS generally requires that you keep all tax liabilities current when you are on an installment payment plan for paying back tax liabilities. If you can show that your current income tax withholding will not result in a tax liability on your 2011 or 2012 tax returns, you may be able to write to the IRS and request that they reconsider the withholding lock as a modification to the installment agreement request. If the lock is creating a current financial hardship, you may be able to contact the Taxpayer Advocates office for assistance and submit Form 911.
For additional information you can download Publication 594 and Form 9465, Installment Agreement Request and its instructions from http://www.irs.gov/
Also, you can locate your nearest Taxpayer Advocates office at this website. http://www.irs.gov/advocate/index.html