2003 transmission

I have a 2003 Z71 with the transmission clunking when it shifts to 3rd and when it down shifts to 2nd or 1st. When it shifts it sounds like you hit the driveshaft with a ball peen hammer. Chevy says it is normal. Having been a mechanic for a lot of years I know they're full of it. It now has 35,000 miles. Has anyone heard of a fix? Chip, change fluid, bump up the pressure, anything to help?
Thanks

The noise you are describing almost sounds like a u-joint going bad. They make that pinging sound when the needle bearings are being crushed in the caps of the u-joints. Just a thought.