My 1988 FJ62 makes a very rough vibrating feeling and loud vibrating sound. It does not happen often but when it does it feels and sounds like you are running over rumble strips on the side of the road. And it sounds like it is mechanical that is either behind the dash or under the floorboard. I've tried to recreate it many of times. If I'm driving about 51-52 MPH and about 2100 RPM while accelerating, I am able to recreate it most of the time. When it happens, I usually just accelerate or decelerate through the narrow window with which it is affected. I've been told that the driveshaft may need to be aligned. Not sure if that is the case because about 6 months ago, I was getting a vibration that reminded me of the feeling of tires needing to be balanced. When I took the car to the shop, I was told that the tires were properly balanced and aligned. They aligned the keys on the driveshaft and the vibration went away. This unusual vibration that I'm trying to diagnose is more of a mechanical sound and feel rather than a tire being improperly balanced feel. Any ideas? Thank you.