Toyota Repair: 1983 Toyota 4 wheel drive pickup, toyota pickup, steering wheel

Question
Recently my son's 83 Toyota pickup started shaking upon reaching the speed of 38-40 mph.  He has the 4 wheel drive that you have to get out and turn the wheels and lock them in when engaging the 4 wheel drive.  About 2 months ago we had transmission rebuilt and brakes replaced.  

Answer
My first reaction is to have the wheels balanced, if it is a shaking that can be felt in the steering wheel.
But there is another possible cause, there is a difference between a shaking/shimmy and a vibration which tends to be more harsh and of a faster fequency, if that is the case the whole truck will vibrate and can be felt in the seat and floorboard at a much higher frequency than roadspeed, in that case look for worn U-joints or an out of balance driveshaft.