Whenever you ‘upgrade’ you move ‘the weak spot’. On a standard axle, front or rear, the weak spot is either your half shafts or the 2 pin diff. So changing the diff for say an 'Ashlocker' means the 'weak spot' is moved - and is now absolutely definitely the shafts. And the 'locker' is going to put far more strain on the shafts than your open diff ever did.
Our recommendation is to do 1 x axle at a time, and do it properly. If you just upgrade both axles with say 'Lockers', and nothing else, you risk the standard shafts exploding under the additional strain being placed on them - potentially then taking out and damaging or even destroying your expensive new Locker.
So, the answer is : back axle 1st, front axle second.
Your rear axle works harder, ie 'foot down' going hard up a hill = all the weight on the rear axle & its components, whilst front axles are normally scrabbling for grip. The front axle however is normally not worked hard when all the weight is on it – ie going down a hugely steep hill may mean the weight is over the front axle, but the power is often not on at all whilst you 'squeak n squeal' a bit …..
The answer is upgrade the rear axle centre AND at the same time fit upgraded 'Half Shafts'. We stock Ashcroft HD Shafts – and 'Drive Flanges' too. Standard 'Drive Flanges' are prone to spline wear and also allowing water in, er - which is not good !. Lastly, add a X-Eng 'Magnetic Drain Plug' (see Off Road Products)
Once this is complete save for the front axle. A rear 'Locker' transforms your off road abilities. A second 'Locker' is not 'twice as good'. Better yes, but not twice as good. Again – centre change ie 'Locker' AND shafts. Unfortunately this may also mean a CV upgrade too, dependant on axle type. KAM & Ashcroft both offer superb solutions, both have pros and cons, and approach the solution quite differently - both are seriously strong. There are such a range of front axles and internal variances that you may find a call to us can save you a awful lot of time and research. Front axles are unfortunately much more expensive to upgrade properly than a rear.