I am having some gearbox problems and the mech is saying that we will have to change it at the end. Is it possible to adapt a 5 speed gearbox to a 2.25 diessel engine.

yes but as the transfer box has center diff and drives both axles in 4wd at all times you either have to put axle in front with cvs ie 109 v8 or change centre diff to 4wd only in difflock and 2wd otherwise you can get kit to do this ,putting defender floor plates tunnel etc improves conversion,there is some work like removing original cross member ,prop modifying etc

Thank you for your advice. But I gues it will be, easyer and cheaper to keep it as 4 speed and adopt a roverdrive to it, from the things you say.