If you jack it up and get some jack stands under it, you could run it up to speed and know for sure if it is coming from the rear.
Road vibration
- Thread starter bwell
- Start date
Yes factory OEM parts. The aftermarket polyurethane support used with a stock driveshaft maybe benefit from tapping ahead 1/8". I believe from an engineering pov when you load the car with passengers and or luggage which would force the driveshaft forward as it travels in an arc, it would relieve or help Center the driveshaft in the rubber or polyurethane support.
Highly dangerous undertaking that should only be performed by an expert. Like yourself James.
There was one member on here that had a vibration and it turned out to be bad brake drums. 
Does the vibration only change with speed of the car or does it change at all with the RPM of the engine? I've seen flex a plate loose a weight once and it made a weird vibration that couldn't be explained. My guess is that center support is the issue or your drive shaft is bent but those issues could have created other issues. Check one at a time like anything else. Always install carrier bearings with the rear axle holding the weight of the car. Check to make sure your drive shaft isn't missing a weight along the entire length. Grab hold of it and see if there is any play and if so where. Rear center section could have bad bearing making your drive shaft play jump rope at 40mph.