Title says it all. I'm pretty sure bent axles/tubes, bad bearings, and maybe even ball joints could cause it. How do I tell which it is?

 

jack up the front of the truck, grab ahold of the tire at 12, and 6, and shake. Does it move? if it does, might be bearings, might be balljoints.

Do the same trick at 9 and 3. if it move, that is prolly wheel bearings.

You can try and measure out the differential tubes.... need a VERY flat surface, and just measure the tubes at various points. All measurements should be the same.