I have a 07 chevy avalanche ltz it ran perfectly fine then one day I was coming up the highway and it quit pulling I was able to roll it to side of the road but it acts like it's in neutral. People tell me that it is the transmission but it gave no warning and it never slipped, whined, or made any grinding noises still doesn't. It also doesn't read any codes. I replace the alternator because I thought that might be the problem and it tested bad. Didn't help still only revs when you change gears. Transmission fluid has no burnt smell or isn't dark colored. I was wondering if it could be something electrical.

 

Take it to a transmission shop to be diagnosed, but it's probably dead.
They just go out eventually. Faster if they're never serviced.

 

Does it shift into any other gear other than drive? Any clunking noises when shifting the shifter through the gears?

I would check the linkage and cable... maybe it slipped off and its in neutral... My 2002 Neon did that once and I freaked thinking it was the tranny that went... but the cable simply slipped off the actuator. Please check it!