So you already have a working 9" diff, that's ½ of the work done, you don't need to buy a whole diff conversion.
Next question would be, are you breaking 28 spline axles? If not then it would be a much simpler process to stick with the axles you have and just buy a 28 spline centre to suit.
If you do want or need to change to 31 spline axles then they will have to be custom made, so you will need to know the length of your current axles and the stud pattern or "PCD" of your wheels. This would involve removing them and measuring.
The next question is, are you happy with the current diff ratio or do you want to change the gear set as well? We can work with you to get the ratio you want but will need details of your rear tyre size and gearbox/speedo setup, then I can crunch the numbers and figure out your target cruise RPM.
Question next, is your current diff carrier (the cast 'pumpkin' that bolts into the diff housing) anything special like an alloy one or whatever?
If not, and you want to change ratios, then I'd suggest buying a second hand carrier and having someone in the city setup the gearing and centre you need. If you do that then you can simply swap the carriers as required, all you need is basic hand tools (and the ability to bench press about 40 kgs)!
Edited by Bigfella237, 02 March 2019 - 06:27 PM.