A g series hilix diff would be nice. U dont want anything too massive depending on what sort of build your going for. But if you move away from certain diff models you may lose aftermarket support for 1.5way or aggressive 2way diff centers. And then you have studd patern so if you have wheels youd probably want to use them. Research carefully on the diff the axle and stub pattern and length. U dont want to pay extra to shorten a diff or whatever witchcraft they pull.

Try upgrading gearbox and diff at the same time so youbl only make 1 custom tailshaft.