Sounds like you may have already given up but I'll throw my 2 cents in anyway for you to think about. I own a Tundra - not a Sequoia - so I've never seen the drivetrain on your vehicle but I imagine their similar. I had a bad vibration that turned into a squeal after it got bad enough (took several months) and that turned out to be the center support bearing on the driveline. Toyota will tell you that slop in this support is "normal" but after it got to the point of producing a squealing noise, they finally decided it wasn't "normal" after all. I too could feel the vibrations anywhere between 45 on up to however fast I was driving. My truck was past the warrantee period but they repaired it under the drive train warrantee and it didn't cost me a dime. If you haven't already, you might want to take a look at that center support (if it has one) and look particularly for excessive slop in all directions, and for any torn rubber in the support sticking out the sides.
Good Luck!
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Factory: Leather, Tow Pkg., Tundra Floor Mats, RS 3000 Security, Six Disc In-dash CD Changer, TRD Off Road Package, Toyota bed extender, auto-dimming mirror, and running boards
Add-ons: INYATI bed liner, Lund hood deflector, Stainless Billet grille, Full tint, Tundra Door sill protectors, Putco s/s bed caps, Delta polished aluminum tool box, Whelen four-way strobe, Bridgestone Dueler A/T Revos 275/70/16's, Throttle Body and Relay/Fuse Billet Covers
Performance: True Flow intake and air filter, 20% Underdrive pulley, Jet Hot SS Autochrome Headers, Power Stop Cross-drilled Rotors & Maxxis Pads, Flowmaster cat-back dual exhaust.
Audio: Alpine V12 amp, Kicker two-way front speakers, Rockford-Fosgate two-way rear speakers, Boston Rally Neo 3r tweets, Phoenix Gold 500.1 sub amp, Twin Rockford-Fosgate 8's in a Q Logic sub enclosure, sound deadening, XM SKYFi2 Satellite Radio
Suspension: Revtek 2.5" suspension lift ,1.5" rear leveling block, Hellwig anti-sway bar w/ ADDCO end links, TRD Red/Blue shocks, TRD Add-a-leafs
Mods: Rear seat angle, Dome/ Dash/Fog/DRL and Cargo Light Mods,Under carriage lights, Ignition Key and Open Door Chime Mods, Silverstar head and fog lamps, Power tailgate lock
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