Drive Shaft!
Check your U-joints...
I had the same problem. One thing that i did notice and have read in other drive shaft related threads is that when you let off the gas pedal the vibration goes away. I didn't see you comment on that.
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02- Limited slushbox, 35" Toyo MT's, @eimkeith Gear, ADS Racing Shocks, Tundra Brake upgrade, SPC UCA's, Yukon 4340 Chromoly rear axles, Metal Tech LCA, SS Trac-bar, 5.29's, Harrop E-lockers F/R and these super sexy Durobumps!
@PlasticsGuy