If its a classic rangey the wheel will be 16 inch not 17, if it is a rangie P38 wheel it wont fit as the PCD is different, you need adaptors which cost a bomb. also on many rangey alloys the centre will not fit onto a defender without a bit of gentle fettling but it can be done, some wheels (if alloy) will not fit a drum braked rear axle, although if you have a defender and not a 90/110 it should be a disc braked rear anyway.
As for gearing the wheel itself has no effect on gearing, we need to know what size the tyre is to see if it will be different.
Cheers Steve
