At the end of year $2,200 of the previously recorded $4,800 Rental Income has not been earned. What is the second account I'm touching (besides Unearned Rental Income.

Accounts receivable/asset. I answered my own question! Let's hope it's right.

I find it best to think through all JE when trying to figure out what the adjustment should be:

The JE that WAS made was:
Dr. Cash/AR 4,800
Cr. Rental Income 4,800

You should have only recorded 2,600

Therefore, you need to:
Dr. Rental Income 2,200
Cr. Unearned Rental Income 2,200