Before you get under the rig, take a look at the videos on the web page I posted. It is not to diffcult to check driveline algnment using a bubble protractor to determine if the u-joint working angles are within specs. They must be within specs or they will not cancel each other. In your question above, checking to see if the driveline is straight from side to side, In some longer wheel base vehicles the driveline can be a compound arrangement. In a compound driveline arrangement the transmission output shaft and the diff. input/pinnion shaft are parallel as seen from a top view and side view, but not inline to each other top and side views. The single plane arrangement the top view is straight from the trans output into the diff. input. The side view has the the two shafts parallel but not straight inline to each other.(see the videos) If the rear axle is not parallel to the front axle your vibration could be caused by this condition. Usually when the front and rear axles are not parallel your vehicle would dog-track, do down the road side-ways. This condition would also cause the front tires to wear the inside edge of one tire and the outside edge of the other tire. If you don't have this tire wear, most likely your axles are parallel. Check all the components discussed above, then check driveline algnment to determine if u-joint angles are within specs.