I found this forum while looking for answers/similar issues.
Just bought a 2009 base model, 4cyl 5spd Tacoma from a reputable Toyota dealer 2 Weeks ago.
After driving a lot of interstate miles, I came to realize it has a constant rumbling vibration from 73 to 76 mph, 4th or 5th gear, doesnt matter. Truck is smooth and tight in all other situations. Sounds and feels like smoother version of driving on the rumble strip on the side of I87.
Going back to dealer @ 20k maintenance/service check. This will be number 1 question for them now.