Check the brake light switch. They're relatively inexpensive, and if faulty will not allow the transmission to shift out of park.
You can also call a local shop for diagnosis!
http://repairpal.com/chevrolet-within-10-miles-of-camdenton-missouri-65020
RepairPal Certified shops can diagnose and repair your 2008 Chevrolet Impala. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.