Vibration from 25-55 mph

I have a 2000 Ford F-350 Turbodiesel, x cab long bed, automatic. The truck has 116k on it. My problem is when driving there is a vibration from 25-55mph. It goes away the higher the speed but is very very bad at those speeds mentioned. I had the tires rotated and balanced after I noticed the problem. I had then put the vehicle in neutral and the ebrake on to check play in the drive shaft- didn't not anything unusual. The u joint closest to the center balance has a good amt of rust powder on the shaft, and the u joints are very dry. Is it possible to have the ujoint make that amt of vibration and then go away with higher speeds? I also looked to see if the drive shaft threw a weight but I didn't notice an area where it was missing anything. Ar there any other areas I should check or anything I may be overlooking? Your suggestions would help.