In my experience, a bent wheel will usually reveal itself a a very low speed, and increase in frequency as speed increases. But don't rule it out. You shouldn't need to break the tires down to identify a bent wheel either. Usually watching them spin on a balancer will show anything bent. Out of round tires would be what I would suspect, being out of alignment for any length of time can play hell with them. Do any of the tread blocks look as though one edge is higher than the other? If you lightly run your hand over the tread surface, does it fell relatively smooth, or does it seem to want to snag and catch your hand on the surface?