Ultimate? For what use?
The conventional wisdom says the R380 is the strongest manual box. What you will need to fit one depends on the engine to which you want to bolt it.
Newer is better on xfer boxes. Stock low range ratios are all the same, high range ratio standard Defender is generally 1.4:1. Disco and RR's got 1.2, so somewhat lower revs in high range. 1.6 is available too. On the bottom end, you can fit an underdrive for crawling if that meets your needs. Axle ratios run the gamut too.
Can you fit TD5 kit to an '88? Sure, anything is possible. What's your skill level, and/or how deep is your wallet?
Not knowing what you want to do with the truck can lead to spending more than you need and not meeting your goals. Better to decide how you intend to use the truck, then ask for help relative to that purpose so others have a context for their advice.
