Sounds like the prop shaft is out of balance, or a UJ on its way out. Either way its just a case of getting the flanges swapped over, any transmission place should be able to do it for you. I just had one done for my Anglia and had a new UJ fitted at the same time, only cost me £20. Worth a try as once you undo the nut in the centre of the diff all the settings go out of the window and it is not an easy job to set up at home.