[alalmcgrath]

The Tundra was stock except for the 20" Fuel Rims and oversized Toyo tires 275/65/20 when I got it. I added the 2.4" Readylift to the front to level the truck out. I didn't put any spacers in the back.

Sounds to me exactly like the infamous driveline "CLUNK". Try searching for driveline clunk or driveshaft clunk. There are plenty of threads on this.