Vibration while driving.

I have noticed a slight vibration with the truck at about 40 mph. Nothing noticeable under or above said speed.
It feels as if coming from the FRT LH wheel. I was told by a "friend" that it might be the wheel bearings (it has 2/wheel in each, right? Inner/outer). Can any one provide some insight?

Since it only occurs at a single sped, I would think it would be a tire weight or out of balance tire/wheel. I would have that tire re-balanced and see if that helps.

You can check for bad bearing by raising the truck on that side and grabbing the tire at 3 and 9 o'clock. Push with one hand and pull with the other, there should be hardly any movement in the wheel, if there is then it is probably the bearings.

Yes there should be an inner and outer bearing in each side. You should replace both sets in each wheel and do the other wheel also.

I would also advise checking that the toe-in is correct, since that adjustment should be made about each 10,000 or 20,000 miles. I dont know if the super-duties use a straight beam axle - but if so it may be the kingpins/bushings should be well lubed (about each 2k miles I think), and if it has a steering box (instead of a rack and pinion), that steering box may need adjustment also. It should be too hard to do, but has to be done in a specific order -loosen pitman shaft, snug up the steering shaft, re-tighten the pitman shaft but dont over-tighten it. This will eliminate the looseness that allows a wheel wobble to get started. When you have the wheel jacked up so you can spin it, you might see some out-of roundness, such as from a tire carcass that had some damaged cords. This would contribute to wobble.
P.S. The speed where you feel tire-out-of-balance is usually between 40-50 mph, depending on the size of the tire and wheel and the weight of the hub they are mounted to. In a small car it would be more like 70-75, as the tires/wheels/hubs are lighter.