2002 Tahoe - clunking noice from transmission

When applying the brakes to my 4x4 automatic transmission 2002 tahoe, it appears that my transmission is "hunting" for first gear. When I am slowing down, I feel the tranmission "let go" of first gear and the truck begins to feel like it's in neutral, then right before I stop, I suddenly feel the transmission "catch" first gear again. When the truck catches first gear, I feel a "shutter and clunk" from the transmission trying to slow down the truck again.

I have tested the transmission in all gears and it only makes the clunking noise in drive and overdrive. I also noticed that is does NOT make the clunking noise in ANY gear when I have the "tow / haul" mode button pressed on the transmission gear shifter.

One GM service department tech told me that they have been having problems with the slip yoke, u-joint and external seal on the 1/2 ton trucks, but that doesn't explain why the tranny does not "clunk" when the "tow / haul" mode is on.

Any suggestions??????