I am trying to stay the same level as my wife as we move through the campaign in S2, but would like to go back to an earlier area for more shards, gold. How much higher do I need to be relative to an area level where I would receive no xp?
You will always get xp. The amount of XP you get is static through the campaign. It’s just that later zones give much more XP than early ones and higher levels (of both your character or your skill) requires significantly more XP to progress.
Also, it’s not a good strategy to go back to farm that. You’ll get way way more as you progress the game.
If your problem is playing on your own when you can’t be playing with your wife, I would advise you to do a separate character for that instead.
Levels are not a meaningful indicator of power in LE. A level 20 character could be as powerful as level 50 character, it all depends on spec and gear.
So I wouldn’t let it limit you too much.
You can also get full XP from up to 10 levels above your current level (lvl 48 player can get full XP from lvl 58 monolith for example).