Ive been experiencing a strong vibration in my truck for the last few weeks now. It is not very noticable in first or second gear, but in third gear the vibration suddenly gets stronger around 2500 rpm, smoothes out a bit, and then gets even worse after shifting into 4th gear(2000 rpm). It is by far the worst while under moderate or light throttle load in 3rd, 4th and 5th gear. If you let off the throttle, the vibration seems to stop, then you put your foot into the throttle and its even worse. I'm no mechanic, but I'm guessing that it is something in the driveline. It had U-joints inspected at the mechanics and he said the U-joints were fine, even though i'm not quite convinced. Rear end maybe? What about a worn ball joint? Any ideas or suggestions would be great.