Tacoma Vibration

My 2002 Tacoma (85K miles, manual trans.) tends to vibrate when I am in fourth gear at 40-45 MPH. it will only vibrate as i accelerate and does not vibrate when i coast or apply the brakes at these speeds. if i had to pick, i would say the vibration can be felt through the steering wheel. It doesn’t vibrate at any other speeds, only between 40 and 45 MPH. have not checked tire rotation, but tires are less that a year old. I do have to drive over washboarded roads every day to get to work, about a mile each direction, and think this may have something to do with it.