If a card or ability says resolve a die showing range or melee damage against an opponent does a modifier die (+2) count as damage?

If a card or ability says resolve a die showing range or melee damage against an opponent does a modifier die (+2) count as damage?

In the wording you used, no. You can't resolve the die by itself, so you would not be able to resolve a modified side. If the card says remove a die showing range or melee and deal that damage to your opponent then yes, you could use the modified side.