PLEASE BEAR WITH ME! I'M REALLY GOING SOMEWHERE HERE BUT THERE ARE A FEW PRECEPTS THAT WE HAVE TO UNDERSTAND. - Tom Woods
Vibrations can be and often are the sum of different sources. Try to imagine this if you will . . .
If you were to graph out a vibration, it will have an amplitude (strength) and a wave length (frequency). There are numerous sources of vibrations in anything mechanical. Everything in the world (probably the universe) has a natural frequency at which it will resonate. Exactly like the tuning fork your music teacher may have used.
Your engine is really pulse driven, each time a cylinder fires, there is a building of torque (2, 3 or 4 times per revolution). The resulting torsional vibrations are somewhat, but not completely absorbed by the mass of the flywheel or the fluid effect of the torque converter. Unless the engine has been precisely balanced & blue printed, there will also be imbalances in the engine because of minor differences in the weight of each piston (4, 6, or 8 times per revolution) non-uniform head volumes and air intake (2, 3 or 4 time per revolution). There is also the likelihood that the flywheel or the torque converter has at least some minor imbalance (once per revolution).
Then there is the possibility that the tires and/or the brake drums have some imbalance. Once per revolution of the tire but a fraction (diff. Gear ratio) per revolution of the engine (high gear, transmission). The tire will also create regular impulses of vibratory forces at a frequency that will be a factor of the tread pattern and the vehicle(wheel) speed.
Then on to the drive shaft. It will also almost always have some minor imbalance(once per revolution of the engine in high gear). Now add in the torsional vibrations that will be there as a result of the drive shaft running through any but a very minor joint angle (2 times per revolution of the drive shaft). This will be compounded with any lift. There are also random patterns of vibrations that will develop from looseness anywhere in the power train.
NOW ON TO THE SIMPLE CONCEPT . . .
If you were to overlay a graph of each of these vibrations you would find some areas where the wave patterns are synchronous and other areas where they would not be (asynchronous). In the areas where the wave lengths are synchronous, the forces created will be the sum of the amplitudes. In the asynchronous areas the forces will be no greater than the strength of the greatest amplitude.
This overlaid graphing will create a regular pattern of increased (strength) vibrations where one could expect what I'll describe as a rhythmic & pulsating (harmonic) vibration. These will occur when the strength and frequency are in the range of the natural resonate frequency of the vehicle.
Disrupting the vibration by applying any number of things, including improper joint angles on the drive shaft may help mask the symptom. I compare this to the doctor who can't figure out what "bug" you have but will cure your snotty nose. Turning the stereo up or a thicker carpet kit may make you happy too, but you will still have the source problem.
There is in fact a foam fill the drive line "trick" referenced in a GM factory service bulletin. This is a referenced as a solution to the problem of a "ringing" noise from the drive shaft. The source of the problem is a result of a ring & pinion mesh or minor torsional vibration problem. Filling the drive shaft with foam simply deadens the noise. Although in the past, I have had problems with filling the drive shaft with foam, it seems that the foam may not harden in the airless environment. I have had better luck cutting the drive shaft apart and packing it with wadded up newspaper.
Think about this: When the door bell rings, is it the striker that makes the noise? , Or is it the chime? This is very similar to the cause & effect that I have attempted to describe above. In this regard, the "solutions of disrupting or masking the problem" are not a cure.
These vibrations, over time, will create fatigue failures. I would recommend that the source/s of the problem be diagnosed and corrected. If this is a Wrangler or TJ,.with any lift, installed or planned, one of the good tail shaft conversions (Currie or JB=), CV drive shaft and adjustable trailing arms be installed. Anything less will be simply "batting your head against the wall", expecting a summary failure that may make the conversion seem cheap by comparison.
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