whirring noise

I have a 2004 Dodge Ram 2500 8' bed club cab pickup 5.7l 4x4 with 54000mi. after I accelerate past 15-20 mph I begin to hear a whirring noise which is fairly constant in pitch, but may decrease in volume slightly when under load. I checked each of the wheel bearings, and all appear to be fine. I checked the U joints which also appear fine. I did notice that the truck has a split drive shaft, and there is what appears to be a rubber guide near the rear of the cab that is worn. could this be the cause of the whirring. any ideas appreciated. Thanks,
Frank