XXXXXXXX,
This sounds to me like the typical GM security problem. They have a resistance value in their ignition keys. The ignition switch has a key sentry connected to it. most dealer will want to replace the ignition key, ignition switch the and the key sentry. Then they have to program the security module to accept the new key. It is not an easy job and you need diagnostic tools with the ability to program. I would take it to a dealer you can trust.
Good luck Mike
ASE Master Technician & T2 diesel
28 years experience. Foreign and Domestic . Ford certified 05-06,Chrysler SC 06-08, GM certified 09