As I read Illinois law on wage attachments, contractor earnings can't be levied against.
The judgment holder can seek to levy your bank accounts or personal property.
Not all earnings can be garnished. Illinois specifically prohibits creditors from garnishing most pensions, welfare benefits, Social Security income, unemployment benefits, black lung payments, alimony and child support, veterans' benefits and unemployment payments. If you make less than $371.25 in disposable income each week, your wages can't be garnished at all.
Likewise, creditors can only levy against wages, therefore independent contractors and other self-employed people are completely immune from wage levy orders.