I have 1984 5.0 GT mustang has been running fine untill a few days ago I get this vibration at 45-55 Mph.had drive shaft checked had bad front u-joint replaced both of them yet vibration is still there when I jack up the car and run it in gear to that speed vibration is there,I marked the flange and pinion flange to make sure I put it back right and not 180 out.my trans yoke (automatic) has a dampner ring does not look damaged .you guys got any suggestions.The vibration goes Hmmm-hmmmm-hmmmm and can feel it through the foor and dash buzzes.Thanks

 

If you go faster does it stop?

 

does your steering wheel vibrate too? it may be your tires are out of balance...but if it is, it is a distinctly mechanical sound?

 

Maybe the car tossed a wheel weight?

 

Yes it stops at about 60 did not take it any faster though. this is not a wheel out of balance .