The auto above i recently purchased ,at 45+ miles an hour a presistant vibration occurs, I removed the driveline, and am replacing the bushing front and rear and center support bearing, Here is the question at the front of the driveshaft between the shaft flange and the flex plate there is another part 123.411.15.87.Looks like a harmonic balancer or a different style flex plate????it doesnt show up in any manuel i own , is it possibe someone replaced the driveline and left this part on in error,???????

I have seen many times where this vibration damper you are seeing breaks apart. Most of the time it is takn off and left off. The part was used only 1 or 2 years and was ineffective.