I have an 03 Yukon with the 5.3 and 460le auto. It shifts harshly from 1st to 2nd after it warms up. I had the dealer drive it and they felt it and suggested a fluid and filter change and grease the slip yoke. That did not fix the problem and now they say the valve body is most likely worn and needs replacing. I think it is 800 bucks for the valve body replacement. Does this sound like it should be worn after 70k? I am not sure what the valve body is either. No codes were present in the ecm they said also.