I am a G.M. tech, if you can get me the codes I can give some help right from the OEM website. The high shift point, and the speedo malfunction sounds like a bad comm. between the speed sensor and ECM/PCM. The place to start is with the codes.

Jim

 

I would be shocked if the O2 sensor problem threw the whole engine off like that, but then again technology is great.... only when it works properly