Help! my trucks making a scary noise.

[Mr99Ranger]

My Ranger has the 5r55e transmission and whenever going up a hill or under heavy load it will make a sort of squealing noise when shifting into range 4 or 5. It shifts fine, though, and has no surge or hesitation. THis high pitched noise is driving me nuts. What could it be?

 

[Quartermile]

Could be the U-joint of the drive shaft going out. Usually starts under load, then the noise starts at lower speeds, and gets worse as you speed up