I know this has been a topic many a time and have tried to use the search bar to get my answer, but I'm looking for a answer I can understand a bit easier...

I want to put Disco diffs and half shafts into my defender to convert the back to disc as well as give a bit more to cruise at 70 on the motorway (I drive a lot of motorway, but don't want a disco... I love my defender!)
I am keeping a eye on fattylewis's thread too.

• Do the diff internals fit into a defender axle, I understand I will need both front and back to keep the same ratios?

• I think the propshaft bolt arrangements slightly different on a disco too?
• do I need to change the entire axle? I think the radius arms are slightly wider on a disco axle?

What else do I need to consider on this upgrade?

much appreciated

EDIT: it's a disco 200tdi and LT77 and I am clueless on tires / ratios (I have some big road tyres on though) so any simple terms are better for me!

I think the defender/disco diffs are the sameratio. Range Rover diffs are a different ratio. The Disco transfer box has higher ratios, which seems to be what you are looking for - a straight transfer box swap.
Early disco rear axles are a straight swop too, if you have early radius arms they may be less thick, so you might need later defender items.

I don't think diffs and diff ratios are different between 90/Defender and Disco, ISTR they're 3.54 to 1 for both ... it's the transfer boxes that are different. 1.4 for Defender, 1.2 for Disco. ish ...

What you want is a Disco transfer box to uprate the gearing. Or change the diff ratios by swapping the crown and pinion gears at each diff ...