The anonymous author referred to Stacy in a two-sentence message as "my love" and thanked her for their wild physical activity.
A private investigator tracked the text message to Sprint Nextel's Web site.
Brodsky says Drew Peterson turned the phone over to police last month.
Stacy Peterson vanished in late October and police say her disappearance is possible homicide.
Drew Peterson has been called a suspect in the case, but hasn't been charged.