Depends on how much work you want to do and how much you want to spend.
I fitted a Disco rear-axle to my 1986 90 some 10-years ago and it is well sorted with the mechanicals being good and recently fitted new calipers, discs & pads. My front axle was also well sorted also being recently fitted with new discs & pads. Swivels etc weren't quite as new but were in first class condition as I had replaced them a while back.
Last year I decided both needed a tidy up paint wise and at the same time I would have had the shock mounts replaced (one hole had elongated) and I was ready to have the casings galvanised but in the end went for replacements instead. My reasoning being that if the axles were new (or as good as new) then they would have little or no wear so I would be set up for many years to come.
New Old Stock TD5 front axles are still available. They come complete with diff, vented discs, calipers, drag-link etc etc (they even come in the crate with oil & swivel grease factory applied) so you just bolt on and away you go.
For the rear that was slightly more difficult. New Old Stock axles aren't available and I didn't want to replace with a used one of similar vintage to what I already had so I went for the latest used 90 rear axle I could find. They are quite scarce (most that are available seem to be 110) and can command a high price but I stuck it out and now have a very recent TDCI 90 rear axle sitting in the garage ready to fit. A quick check over and new pads fitted (don't know why as the originals were hardly worn-in) and it will be fitted to the vehicle in the next week or two.
What will fit?
Defender, Disco-1 or RRC. You may need to replace the diff flange if using a Disco-1 axle but other than that it goes straight on.
If you currently have narrow radius-arms on the front and you fit a later axle then you will need to replace the arms as well but other than that it all just bolts together, even the brake-pipes match. On the rear it is even easier as the trailing arms are the same size, I just had to replace the duel-line brake system for a single line but it took only minutes as the fittings are the same.
Check not only the oily bits but the casing as well. Diff pans rust, mountings rot and holes elongate (as mine had) so it might be worth stripping down, welding as necessary and then painting (or galvanising) so you have a casing that will last a long time.
Both my replacement axles have anti roll-bar mountings but I don't have those on my early vehicle so they are not used. The TDCI axle has ABS sensors but they are just disconnected.
