I've narrowed down the buggy gearbox problem to the diff. I can select all 4 gears (phew), but cannot get drive to the final drive. When in gear, the pinion turns the crownwheel fine, but the diff does not transfer this to the final drive. Is it likely that the diff is buggered? Can it be fixed? Is it a DIY job or does it need to go to a gearbox specialist?