if its internal you can tell by putting your ear to the handle of a long screwdriver or pri-bar if the sound is transferred thru lika a stethascope youll know otherwise itrs probally the wires

 

when its dark out. pop your hood just enough so that you can stick your head under without the light coming on. when you have an effed up wire it usually arcs (bluish spark).

 

If it is wires, it seems like I would always hear the noise, not only 40 MPH and up. Also, The noise is rotation based, not RPM based.

 

youve checked all of that so this is going to sound retarded, have you checked the tires for rocks, nails and screws?

 

I have checked everything so far except for using the listening rod on the front axle housing. I will try to do that this weekend. As far as checking the tires, I pulled off on the side of the road right after I noticed the sound, and pulled out every little pebble in them. I didn't see a nail either. If it is the wheel bearing, are they replacable on this truck, or is it the whole assembly that has to be replaced?

 

the hub bearing can be replaced.

 

you said its rotation based and its prominent over 40mph how do you plan on listening to the axle with screwdriver?

 

He'll have to either run really fast or have a super long screwdriver.

 

You could bungee cord yourself under the truck and have a friend drive the truck!

 

Sure... Either that or put it on jackstands in 4 wheel drive.