I just bought this truck. It has 120,000 miles on it and it is making a whining noise that is audible once you get the truck up to about 55 miles per hour. When you let off the accelerator it goes away and then returns as soon as you press the gas pedal. Any idea what this might be? I just spent a fortune on having the front-end worked on and am afraid of what I might hear at a transmission shop. Thanks for the advice. I did replace all the fluids with Redline. It didn't make a difference.

sounds like a ring gear....ring and pinion is very high...900 or up i'd imagine...

i'd try to live with it if its not too bad...or look for a used unit