steering wheel vibration on 87 GMC van

I have an 87 GMC full size van. There is a vibration in the steering wheel between 35 and 45 miles per hour. It has a different intensity when you are accelerating than when you are coasting (much wrse when accelerating). I have had the front suspension checked and it is good. Could this be a tire problem? A bad engine or transmission mount? A bad u-joint?
Any ideas? Please let me know.