Hi,
the axles from a 2003 model are from a disco 2 which will require a great deal of modification to the vehicle to fit. Your looking for disco 1(upto 1998 you have to change the rear diff flange to suit the 90 prop) or a later range rover classic are the ones you want and will fit straight on.
You will need to use the front brake calipers from the 90 and use the front radius arms from the disco or rr as these will be wide bushes at the axle end, your 90 will currently have narrow bushes at the axle end.Well worth the work to gain disc brakes on the rear axle.
cheers,
Stevie.