It's been a hell of a long time since I worked on a MK 1 escort, so my memory might not be that good, but, does it have a 1 or 2 piece propshaft. If it's a 2 piece (fitted with center bearing) then it's probably the same problem the MK 1 and 2 Granada's suffered from.If it's a 2 piece prop, what you can do is fit some shims or washers, (about 1mm thickness) between the bearing mounting plate and the floorpan. Start off with 2mm worth of shims/washers, and drive the car, if it gets better, keep adding shims until the vibration goes. If it gets worse, remove all the shims and then do the same to the gearbox mounting plate and see how it goes. What this does is basically alters the driveline angle. If it's a 1 piece prop, them just try at the gearbox mounting plate. Another thing to consider is to check the balance of the propshaft and check the UJ's on the propshaft as well.