Driveline lash??

My '04 Silverado 4x4 automatic has made clacking and clunking noises when shifting gears since it was new. Been to dealer yada yada yada no help there. I can produce the clunking at will now. It always happens when Im slowing down to make a right or left down, the tranny shifts into 2nd then as I make the turn and it shifts back into 3rd CLACK! I can produce the same sound just from switching from drive to reverse and back in my driveway, but its not quite a loud as when Im moving. It seems to be the loudest when I've slowed down, made a turn, mostly to the right and then accelerated. Is there any idea what this is out there?? Is it the infamous slip yoke I've read so much about? Or is it normal driveline lash? I've been annoyed to no end with this crap since I drove it off the lot 33000 kms ago, any help with this is much appreciated.