You would disclaim if there is a major scope limitation or lack of independence on the auditors part. You would withdrawl if management is unethical, fradulent, deceptive or completely unwilling to make changes to their financials to clear up mistatements.

I always think of it as a disclaimer relates to an auditor not being able to perform, whereas a withdrawl would be caused by management. Hope that makes sense, I thought Becker covered it well......