Humming noise when driving....

When I accelerate there seems to come from under the cab a humming noise or a "whinning" sound that when I go fast it hums even faster and when I slow down it hums slower and this noise it apart from the normal sounds that the truck should make. sounds like there are new thresds on the tires but there isn't any. When Im at an idle and step on the gas pedal its a normal sound but when I go the humming starts up again. Could it be from the 4wd switch being turned when someone was driving it at a higher speed? My wife had for the week and she brought it back to me like this. pissed me right off. any help will be appriciated. thanks.

I had a noise like that in my 96 caused by a U-joint going bad in the drive shaft - it was pretty easy to diagnose just by comparing it to the others.