Zoloft withdrawal can cause agitation and insomnia. So I suppose, your panic attacks may be aggravated by withdrawal.
You can practice relaxation techniques (e.g. progressive muscle relaxation) to help relax body and mind. Herbs with sedative properties also may slightly help to relax (list of herbs used for insomnia).
If you don't believe in non-medical approaches, you can talk to you doctor about drugs. Benzodiazepines are usually very effective for both panic attacks and insomnia, but are highly addictive. SSRIs are also used for panic disorders, but since you are discontinuing Zoloft probably SSRIs are not an option.