Go ahead and finish Forgotten Gods on Ultimate and get to 100, which will enable you to farm anywhere at scaled max level.
Shattered Realm scales up in difficulty the farther you go so you can use it as a yard stick for how comfortable you are with your build.
Just after finishing campaign the easiest way to gear up will be monster totems and the skeleton key dungeons like the Bastion of Chaos or Port Valbury - they come with plenty of MIs and legendary drops and can be easier than Shattered Realm at first.
Crucible can also be a fun diversion too if you want to shake things up from campaign content.