Steering wheel clunking ,

I have a 1999 Freightliner Century,It has a stationary steering column , when I drive over rough or uneven terrain especially at slow speeds I feel a clunking in the steering wheel and at times the wheel becomes harder to turn.I noticed yesterday when it does this if I pull on the top of the steering wheel it will clunk and then become easy to turn again.The truck drives fine does not pull or shimmy the steering is fine it's just this clunking and binding that appear to be coming from the steering wheel shaft itself.I checked the shaft U joint by the steering gear it needed tightened but now it seems worse,is their another U joint under the dash or bushing somewhere.