I've got a crazy problem I can't figure out

[crawdad]

When I'm going real slow in my truck, I can feel, what seems like, a dragging brake. It is definitely related to the speed of the truck, but I can't feel it at high speed. I first noticed it when I was rolling down the driveway real slowly. It feels just like the brakes are grabbing with each turn of the wheel. So, I've put new tires on the truck, new ball joints installed, front end aligned and have had the brakes replaced front and back. and it still does the same thing! I don't know what else could cause it. Could a transmission cause this. Tranny shifts fine and I haven't noticed a problem with it. I'm at a loss and could sure use your ideas on what to look for. Thanks.