Edited to say I disagree with some of mmgemini's points
1. Change to the Wolf axle was with the 2002 model year, early 99-01 Td5s had the Salisbury rear axle fitted because we've got a few at work. The new axle came in at the same time as the new dash with the big square box for the radio, switches etc. and at the same time there were other minor changes to brakes, panhard rod and probably some other tweaks which I forget.
Disc brakes on the rear axle came in for the last 9000 or so late production 200Tdi Defenders from (guessing) some time in 1993, and from then onwards, all 300Tdis and Td5s have discs all round. You will find a late 300Tdi with the new Wolf rear axle fitted, because I have got one, but by "late" I am talking about 2006 export spec 300Tdi, not anything available in the UK
2. Under the back on all of them, plastic tank on later models which is better/worse in various respects. Underseat tank not possible/easy on 1999 onwards Td5 models due to underseat ECU and fuse box!
3. No