If the vibration comes from the seat, its probably chassis related ( meaning everything except the steering).
If the vibration is felt in the steering, next question is .
Does it do it braking (no?), then its not the brakes.
Does it do it at all speeds, check tires rims and steering components.
Does it do it only at certain speeds ( the harmonic range).
You might want to sit down while reading this.
I find it amazing that nobody has made this comment yet...so keep reading...
The 1st generation , 2nd generation and some SOME (SOME), 3rd generation Tacos.
Have a bad fitment between the front driver side axle and the differential housing bearing.
This is a known issue for some of us with the older tacos (re: tacomaworld.com)
The good news is that there is a solution...
You need to have the bearing replaced with a bushing made by ECGS.
(East Coast Gear Supply).
They also sell the installation tools to do the job ( about $150), complete.....a case of beer with a good buddy and a sunny afternoon.
The vibration issue is even more pronounced as the truck ages or has a lift kit,...or both....
Known issue,...known issue...known issue....
For the few that do vibrate, Toyota play dumb.
They know about it and why its happening, but the cost to correct this is too expensive.
You , mister consumer are now a Toyota replaceable commodity.
GM once thought like this.
And they eventually needed your tax dollars to keep from sinking like a rock.
Toyota will eventually have the same fate.
Shareholder return/falling sales = history repeating.