Hi and welcome. The most easily available is the ARB for sure, but is a pricey item and best fitted by someone with the right skills too. If you have seen one for sale, that is a rare beast indeed. I had one and there is one other member IIRC on here who has one. Of course they could be wrong and THINK it has three lockers so be careful. If they didn't want much for the truck and it had an ARB fitted, buy the truck, pinch the axle and break the rest. A front diff assembly on its own with ARB fitted has to be worth best part of a grand.
In terms of going a different way, I am not sure that there is much out there. The 80 has solid front and rear axles with electric lockers and the 90 has independent front suspension. IFS. Having had both vehicles, I am not sue that between us all, on good day, that we could fit an 80 locker to a 90. With enough skill, I believe that you could take a front axle from an 80 (which is a 90 back axle basically) and fit that to a 90 so that you had live axles all round. I have seen that done. But it's a huge undertaking for what is comparatively little gain. It would be easier to just buy an 80 to be honest. I fitted one because I wanted to be one of the first. It didn't make a staggering difference to the off road ability. Especially on a short wheelbase model. You would be better spending the money on a winch and winch mount system.
Chris
