4L60E is the transmission. I don't know much about them, but have read they are not the most reliable transmission out there. First thing is to check and make sure it's full of fluid. Is your check engine light on? I know some of the electronics on these trans will throw lights and may make it a little easier. There are a bunch of forums out there (fullsizechevy.com, gmt400.com etc) that can probably give you better information.
