Sheltered by Ghosts grants one too many benefits for its CMC and rarity in my opinion. When you look at other uncommon 2 CMC auras, as well as non-aura enchantments that exile, they grant less by comparison.

Example: Feather of Flight grants +1 power and a creature effect (Flying vs Lifelink), so we're comparing drawing 1 card at instant speed with a nonland exile and Ward 2.

Example: All that Glitters grants +1/+1 for each artifact and/or echantment you control. It makes the creature stronger for combat without affecting the opponent's board or reducing its susceptibility to being targetted by spells and abilities. This is significantly less than Sheltered by Ghosts grants.

Example: Sentinel's Mark grants +1/+2 and Vigilance permanently at instant speed, and Lifelink until end of turn on the condition you cast it on your turn. This is basically Sheltered by Ghosts but trading instant speed and temporary, conditional Lifelink for sorcery speed, permanent Lifelink, a nonland exile and Ward 2. This makes Sheltered by Ghosts seem extremely unbalanced by comparison.

Example: Trapped in the Screen exiles a nonland permanent and has Ward 2, but it's 3 CMC, common and not an aura. If Trapped in the Screen were changed to an aura, that alone would warrant either the CMC being reduced to 2 or rarity increased to uncommon, but also granting +1 power and Lifelink should increase the CMC back to 3 or increase the rarity to Rare.

This, among other reasons and examples, is why I believe Sheltered by Ghosts grants one too many benefits for its CMC and rarity and should be banned.