you either have a defective shifter assembly or rod that
goes to the shifter. either way, this is not a repair for a novive, you will need a reputable GM tech to diagnose this for you.
RepairPal Certified shops can diagnose and repair your 2006 GMC Envoy. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
I called and spoke with Robert on an issue similar to this, but on a 2004 Envoy and he was VERY helpful!