I've just done a gearbox on a Mk3 mondeo 1.8 zetec and that did exactly the same thing. After a lot of head scratching it turned out to be the driveshafts hadn't gone back into the gearbox properly even though I thought they had. All I did was jack the front up loosened the Driveshaft nuts at the wheels and gently tapped with a hammer then did the nut up and it drove straight away. Exactly the same prob as yours. When one wheel was off the ground one wheel would go round but could be stopped by hand.