2003 Ford F-150 vibration

I suddenly have a terrible vibration at speeds above 50mph. I feel it in the steering wheel and gas pedal and can hear it. It seems to vibrate less when I let off the gas. I had a little play in one wheel bearing so I changed it but it did not help. The other wheel bearing is fairly new also. The tires don't appear to be worn, they are less than a year old. Any ideas?

thanks

Chad Reynolds