If you want the e- lociking diffs that fzj80 has.
You can swap them into Fj40 axles provided that you have full floating rear axles, but you need different axles shafts, the longer one in the front, and the short one in the rear . Also you 2 rear e-locking diffs from the fzj80. Not all 80s have lockers. There are also cable conversion kits you can get for these locking diffs