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