Thanks RustyRatRod , ok Red63 i'll give a list of things to visually look for and then some driving feel and then if that doesn't help find the issue i'll need you to relay exactly what you feel as it starts, in which gear, at what speed, when it gets worse, or better and with as much detail as possible as if i was sitting driving it myself.
Was anything changed recently and this vibration started ?
OK, 1st you can jack it up at the rear , jack stands under the rear axle , put the car in neutral (leave the car off) and at the front of the shaft hold the slip yoke and the shaft and look to see if you have any movement in the joint by twisting the two pieces opposite each other, if nothing then do the same at the rear diff and hold the end yoke and shaft and again twist them opposite each other, no movement is good.
Then remove the shaft and see if the joint moves freely and isn't seized on either plane , it sounds like redundant checking but you can have a seized joint or a worn joint and sometimes the worn joint can be missed immediately removing it from the car unless it is that worn, but if it's slight it's easier to see.
If you have no movement and no seized joints put the shaft back, if you have a seized joint replace it, if it's at the slip yoke mark the slip yoke to the shaft, this is how the shaft was balanced and that is how the slip must be put back to the shaft.
If you are passed that and neither of those scenarios of joints were an issue then you need to drive it, just drive it normally to the speed it starts and try and stay at the speed it is most violent, at that speed drop a gear and take notice of the vibe has any change at all, if the vibe changes at all you'll need to know if it got faster or seemed less intense, if it got faster the vibe isn't at the shaft or rear axle or wheels, if it got less intense somewhat faster but is still there somewhat it could be pulse related and not driveline, if no change at all then we need to go back to the gear you were in and lets go faster and see what happens (in the driveline).
At this point we are looking to see does the vibe come back at 70 or 80, or 100 or not at all. We'll also go back to that speed where it starts and put it under hard load (floor it) and see what it feels like or doesn't feel like. Also at that speed you'll need to notice if in turns does it change at all.
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