I have been getting a slight vibration that I can feel in the floor board (and can hear) at all speeds but seems to feel more "constant" at higher speeds (60 MPH and up). IT sounds like I am driving over a small rumble strip but at 45 MPH it is on a regular pattern of 4 to5 second intervals. I have tried several things:

New tow alignment
Balanced wheels
Checked run-out on rear drive shaft.
Checked for loose transmission mounts.

I would have guessed that it was drive shaft, but the shaft appears to be fine and the run-out checks out fine as well. U-joint?

The truck has 97,000 miles.

The "rumble" can be felt and heard as low as 20 MPH but tends to go away at highway speed. It is definately tied to the speed of the vehicle. Also I have put the truck in neutral and the rumble is the same. It does not follow the engine speed.
I thought it might be bearings, but I do not hear anything when I turn. It is not linked to a side load.

I do not feel the truck vibrate, I just hear it and feel it in my left foot through the floor board. Yes, I am picky.

Thanks for the help.