OK, no jokes from the boys please...

Seriously....
When I let off the gas at 30 mph - 80 mph, I feel a vibration from the front of the car. I don't think it's the tires/wheels (although they're aftermarket) because it only does it when I let off the gas & coast. I feel it in my pedals and my seat. I had a similar problem with my Ram Pick-up. I think that was the U-joint.
Any suggestions?