My 2001 f-150 make a high pitch whine when driving over 40mph. It sounds like it's coming from the rear of the truck. Rear differential? Drive shaft? U-joints? Does something need lubrication?

thanks for your help.

 

I get that sound as well when my wifes rides in the back.

Have you checked the rear diff fluid level?
Did you do a visual to see if there is 'wetness' around the diff?
No clunking from drive to reverse?
How many miles are on the truck?
Is it a free rear or locking?