I have a problem but I don't know what
I have a 2004 F250, 6.0L. My dad borrowed it to pull his camper. He brought it back and said that the trannie was acting up, slipping, not shifting up properly, hanging up in higher gears when slowing, etc. When we checked the trannie fluid it was low but not empty or dangerously low. We added some Lucas Trans treatment to see if this would fix the problem. My dad drove it again the next day and said that it was still not working properly. I drove it to work on Saturday, a distance of about 27 miles one way. About half way, on the way there, I was stopped at a traffic light. When the light changed and I went to pull off the motor started making a woo sound as if it was under a lot of stress and strain as if the trannie wasn't shifting up through the gears. It also seemed that their was very little power and that the throttle was hard to push down on. The boost gauge was registering boost so I'm guessing that it wasn't and issue with the turbo. When I started to pull off from the next light that I was stopped at I listened very closly to the shifting of the trannie and it seemed, from the sound of the RPMs rising and falling, that the trannie was shifting without any issue. However, the tact on the dash was showing that it was turning 2500 to 3000 RPMs and only running at about 45 mph and the motor was still wooing and sounding as if it were under a lot of stress. When I arrived at work I popped the hood, got out and listed to the motor. The woo sound was still present but it was very faint, almost like you had to be listening specifically for the woo sound to hear it at all. This is my first diesel and I don't know a whole lot about them yet so I'm hoping someone here can help me diagnose the issue. Please, if you need any more info to help possibly diagnose the issue please feel free to ask. Thanks in advance for any and all assistance.